LaTex Table Font Size Too Small

It still needs more adjusting but thi smay get you closer

documentclass[a4paper]{article}

usepackage{array,longtable}

begin{document}
%begin{table}[htp]% not !h  except in extreme circumstances [h!]

small
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begin{longtable}{@{}>{raggedright}
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@{}}
textbf{Authors} &
textbf{Type} &
textbf{begin{tabular}[t]{@{}c@{}}Fuel and Engine  Specificationend{tabular}} &
textbf{Research Focus} &
textbf{Outcomes}  hline
endhead 
Karag et al. {[}ref{]} &
E &
Hydrogen-Diesel PFI &
Impact of Hydrogen Proportion on BTE &
Hydrogen injection in PFI reduces volumetric efficiency and BTE and also increases heat loss to cylinder walls due to low quenching distance of hydrogen  
Menaa et al. {[}ref{]} &
N &
Hydrogen-Diesel PFI &
Impact of Hydrogen addition on back-fire and BTE &
Validated results of Karag et al. by measuring in-cylinder wall temperatures which  increased as well as 3-D output of in-cylinder equivalence ratio's which were high near cylinder waslls  
Christodouluo and Megaritis {[}ref{]} &
E &
Hydrogen-Diesel PFI &
Impact of hydrogen addition on  the emissions and combustion  of a diesel engine &
Increased in-cylinder pressure at ignition timing due to explosive-type  combustion observed as a consequence of hydrogen  laminar burning velocity.  Heat release rate results also increased with  higher hydrogen addition due to the aforementioned properties  and resulted in shorter combustion periods, taken as CA90-CA10 
Zhou et al. {[}ref{]} &
N &
Hydrogen-Diesel PFI &
Impact of hydrogen addition on  the emissions at varying loads &
Hydrogen addition is effective in reducing  CO2 and CO emissions at all loads. NOX emissions are higher than a diesel engine  but relatively lower at low engine loads  
Nag et al. {[}ref{]} &
E &
Hydrogen-Diesel PFI &
Impact of variable EGR  on emissions at variable  hydrogen addition &
EGR rates of 0-10% reduce BTE by 5-7% due to  lower oxygen availability in the intake. EGR also increases CO due to incomplete combustion caused by lower in-cylinder temperatures. EGR decreases NOX emissions due to lower  in-cylinder temperaute; however, ppmV are still higher than minimum regulations  
Saravanan  et al. {[}ref{]} &
E &
Hydrogen-Diesel PFI &
Impact of urea SCR on emissions &
Reductions of up to 74% in NOX emissions were possible. However, ammonia slip into exhaust was a fundamental issue as well as the operating life of the catalyst.  
Kamil and  Rahman {[}ref{]} &
E &
Hydrogen-Gasoline PFI &
Impact of Hydrogen addition on gasoline PFI SI engines &
Small hydrogen addition of up to 10% favourably increased LFS  ; however, similar to PFI Diesel engines BTE was reduced due to  lower volumetric efficiency. Higher combustion efficiency was  experienced ascribed to higher in-cylinder temperature  promoting operation near-ideal otto-cycle   
Wang et al. {[}ref{]} &
E &
Hydrogen-Gasoline PFI &
Impact of equivalence ratio on hydrogen-gasoline combustion dual  fuel SI engines &
Lean fuel combustion at an equivalence ratio of 0.7  attained stable and efficient combustion due to hydrogen's fast  LFS and flammability ratios. COV was found to be lower up to 10% hydrogen addition since combustion duration is shorter  
Ji and Wang {[}ref{]} &
N &
Hydrogen-Gasoline PFI &
Impact of equivalence ratio on dual fuel SI engine emisssions &
Carbon monoxide emissions decrease fundamentally from  stoichiometric to lean since more oxygen is available at lean conditions to oxides CO to CO2.  CO2 emission also decreases  with lean operation due to lower specific fuel consumption.  NOX emissions increase to an equivalence ratio  of 0.85 and decrease beyond that value  
Yu et al. {[}ref{]} &
E &
Hydrogen-Gasoline DI &
Impact of hydrogen direct injection  strategy on characteristics of lean burn hydrogen–gasoline engines &
Hydrogen injection pressure has more impact than injection timing. With 4MPa and 5MPa having the highest IMEP since they form hydrogen distribution that is more easily ignited. Also, late  injection timing lowers IMEPas it allows for a more  homogenous mixture that does not form a stable flame kernel  
Yu et al. {[}ref{]} &
N &
Hydrogen-Gasoline DI &
Impact of hydrogen direct injection on  hydrogen mixture distribution, combustion  and emissions of a gasoline/ hydrogen  SI engine under lean burn condition &
Validated numerically the results of Yu et al. using a CFD software. However, since injector was located near intake port, early injection resulted in higher in-cylinder pressure and efficiency due to more concentrated mixture near the spark plug. Emissions also decrease with injection timing due to more homogenous mixture allowing for complete combustion  
Elsemary et al. {[}ref{]} &
E &
Hydrogen-Gasoline DI &
Impact of SIT on combustion, efficiency and emissions &
Advancing SIT towards TDC realised lower fuel consumption and higher BTE since more work is done during the power stroke. However, carbon monoxide emissions increase with late SIT caused by lower combustion durations increasing incomplete combustion.  
Duan et al. {[}ref{]} &
N &
Hydrogen PFI SI &
NOX missions in a Hydrogen Fuelled  PFI SI Engine &
NO constitutes 97% of all NOX emissions due to increased temperatures, NO further decomposes into NO2 after combustion  when temperatures cool down. Furthermore, thermal NO was most prominent at 70% due to in-cylinder temperatures while prompt NO was limited due to lean-fuel operation  
end{longtable}

normalsize

end{document}

documentclass{article}
usepackage{makecell}
usepackage{booktabs}
usepackage{pdflscape}
usepackage{xltabular}
usepackage{ragged2e}
newcolumntype{L}[1]{>{raggedrightarraybackslash}p{#1}}

begin{document}
begin{landscape}
begin{xltabular}{linewidth}{L{2.25cm}cL{2cm}L{3cm}X}
caption{caption text} label{tab:key}
toprule
thead[l]{Authors} &
thead{Type of Analysis} &
thead{Fuel & Engine  Specification} &
thead{Research Focus} &
thead[l]{Outcomes} 
midrule
endfirsthead
caption{caption text - continued from previous page}
toprule
thead[l]{Authors} &
thead{Type of Analysis} &
thead{Fuel & Engine  Specification} &
thead{Research Focus} &
thead[l]{Outcomes} 
midrule
endhead
bottomrule
endfoot
bottomrule
multicolumn{5}{l}{footnotesize Exp. = Experimental, Num. = Numerical}
endlastfoot
Karag et al. {[}ref{]} &
Exp. &
Hydrogen-Diesel PFI &
Impact of Hydrogen Proportion on BTE &
Hydrogen injection in PFI reduces volumetric efficiency and BTE and also increases heat loss to cylinder walls due to low quenching distance of hydrogen  
addlinespace
Menaa et al.{[}ref{]} &
Num. &
Hydrogen-Diesel PFI &
Impact of Hydrogen addition on back-fire and BTE &
Validated results of Karag et al. by measuring in-cylinder wall temperatures which   increased as well as 3-D output of in-cylinder equivalence ratio's which were high near cylinder waslls  
addlinespace
Christodouluo and Megaritis {[}ref{]} &
Exp. &
Hydrogen-Diesel PFI &
Impact of hydrogen addition on  the emissions and combustion  of a diesel engine &
Increased in-cylinder pressure at ignition timing due to explosive-type  combustion observed as a consequence of hydrogen  laminar burning velocity.  Heat release rate results also increased with  higher hydrogen addition due to the aforementioned properties  and resulted in shorter combustion periods, taken as CA90-CA10  
addlinespace
Zhou et al. {[}ref{]} &
Num. &
Hydrogen-Diesel PFI &
Impact of hydrogen addition on  the emissions at varying loads &
Hydrogen addition is effective in reducing CO2 and CO emissions at all loads. NOX emissions are higher than a diesel engine  but relatively lower at low engine loads  

Nag et al.{[}ref{]} &
Exp. &
Hydrogen-Diesel PFI &
Impact of variable EGR  on emissions at variable  hydrogen addition &
EGR rates of 0-10% reduce BTE by 5-7% due to  lower oxygen availability in the intake. EGR also increases CO due to incomplete combustion caused by lower in-cylinder temperatures. EGR decreases NOX emissions due to lower  in-cylinder temperaute; however, ppmV are still higher than minimum regulations addlinespace
Saravanan  et al. {[}ref{]} &
Exp. &
Hydrogen-Diesel PFI &
Impact of urea SCR on emissions &
Reductions of up to 74% in NOX emissions were possible. However, ammonia slip into exhaust was a fundamental issue as well as the operating life of the catalyst. addlinespace
Kamil and  Rahman {[}ref{]} &
Exp. &
Hydrogen-Gasoline PFI &
Impact of Hydrogen addition on gasoline PFI SI engines &
Small hydrogen addition of up to 10% favourably increased LFS  ; however, similar to PFI Diesel engines BTE was reduced due to  lower volumetric efficiency. Higher combustion efficiency was  experienced ascribed to higher in-cylinder temperature  promoting operation near-ideal otto-cycle addlinespace
Wang et al. {[}ref{]} &
Exp. &
Hydrogen-Gasoline PFI &
Impact of equivalence ratio on hydrogen-gasoline combustion dual  fuel SI engines &
Lean fuel combustion at an equivalence ratio of 0.7  attained stable and efficient combustion due to hydrogen's fast  LFS and flammability ratios. COV was found to be lower up to 10% hydrogen addition since combustion duration is shorter addlinespace
Ji and Wang {[}ref{]} &
Num. &
Hydrogen-Gasoline PFI &
Impact of equivalence ratio on dual fuel SI engine emisssions &
Carbon monoxide emissions decrease fundamentally from  stoichiometric to lean since more oxygen is available at lean conditions to oxides CO to CO2.  CO2 emission also decreases  with lean operation due to lower specific fuel consumption.  NOX emissions increase to an equivalence ratio  of 0.85 and decrease beyond that value addlinespace
Yu et al. {[}ref{]} &
Exp. &
Hydrogen-Gasoline DI &
Impact of hydrogen direct injection  strategy on characteristics of lean burn hydrogen–gasoline engines &
Hydrogen injection pressure has more impact than injection timing. With 4MPa and 5MPa having the highest IMEP since they form hydrogen distribution that is more easily ignited. Also, late  injection timing lowers IMEPas it allows for a more  homogenous mixture that does not form a stable flame kernel addlinespace
Yu et al. {[}ref{]} &
Num. &
Hydrogen-Gasoline DI &
Impact of hydrogen direct injection on  hydrogen mixture distribution, combustion  and emissions of a gasoline/ hydrogen  SI engine under lean burn condition &
Validated Num.ly the results of Yu et al. using a CFD software. However, since injector was located near intake port, early injection resulted in higher in-cylinder pressure and efficiency due to more concentrated mixture near the spark plug. Emissions also decrease with injection timing due to more homogenous mixture allowing for complete combustion addlinespace
Elsemary et al. {[}ref{]} &
Exp. &
Hydrogen-Gasoline DI &
Impact of SIT on combustion, efficiency and emissions &
Advancing SIT towards TDC realised lower fuel consumption and higher BTE since more work is done during the power stroke. However, carbon monoxide emissions increase with late SIT caused by lower combustion durations increasing incomplete combustion. addlinespace
Duan et al. {[}ref{]} &
Num. &
Hydrogen PFI SI &
NOX missions in a Hydrogen Fuelled  PFI SI Engine &
NO constitutes 97% of all NOX emissions due to increased temperatures, NO further decomposes into NO2 after combustion  when temperatures cool down. Furthermore, thermal NO was most prominent at 70% due to in-cylinder temperatures while prompt NO was limited due to lean-fuel operation addlinespace
end{xltabular}%
end{landscape}
end{document}

Here's a solution that employs an xltabular environment and variable-width X-type columns. The full table spans a bit more than 2 pages; the following screenshot shows just the first few lines. I would also recommend using the line-drawing macros of the booktabs package -- such as toprule, midrule, and bottomrule -- instead of hline.

documentclass{article} % or some other suitable document class
usepackage[margin=1in]{geometry} % set page parameters suitably
usepackage{xltabular,ragged2e,amsmath,booktabs,mhchem}
newcolumntype{L}[1]{>{RaggedRighthsize=#1hsize}X}
newcommandmytab[1]{begin{tabular}[t]{@{}l@{}} #1 end{tabular}}
begin{document}

setlengthtabcolsep{4pt} % default: 6pt
begin{xltabular}{textwidth}{@{} lll L{0.667} L{1.333} @{}}
toprule
Authors &
mytab{Type ofAnalysis} &
mytab{Fuel and Engine  Specification} &
Research Focus &
Outcomes  
midrule
endhead

bottomrule
endlastfoot 

mytab{Karag et~al.  {[}cite]}
& Experim. &
mytab{Hydr.-Diesel PFI} & 
Impact of Hydrogen Proportion on BTE &
Hydrogen injection in PFI reduces volumetric efficiency and BTE and also increases heat loss to cylinder walls due to low quenching distance of hydrogen  
midrule
mytab{Menaa et~al.  {[}cite]}
& Numerical &
mytab{Hydr.-Diesel PFI} & 
Impact of Hydrogen addition on back-fire and BTE &
Validated results of Karag et~al. by measuring in-cylinder wall temperatures which increased as well as 3-D output of in-cylinder equivalence ratio's which were high near cylinder walls  
midrule
mytab{Christodouluo and Megaritis {[}cite{]}}
& Experim. &
mytab{Hydr.-Diesel PFI} & 
Impact of hydrogen addition on the emissions and combustion of a diesel engine &
Increased in-cylinder pressure at ignition timing due to explosive-type combustion observed as a consequence of hydrogen laminar burning velocity.  Heat release rate results also increased with higher hydrogen addition due to the aforementioned properties and resulted in shorter combustion periods, taken as CA90-CA10  
midrule
mytab{Zhou et~al.  {[}cite]}
& Numerical &
mytab{Hydr.-Diesel PFI} & 
Impact of hydrogen addition on  the emissions at varying loads &
Hydrogen addition is effective in reducing ce{CO2} and ce{CO} emissions at all loads. ce{NOX} emissions are higher than a diesel engine but relatively lower at low engine loads  
midrule
mytab{Nag et~al.  {[}cite]}
& Experim. &
mytab{Hydr.-Diesel PFI} & 
Impact of variable EGR on emissions at variable hydrogen addition &
EGR rates of 0--10% reduce BTE by 5--7% due to lower oxygen availability in the intake. EGR also increases ce{CO} due to incomplete combustion caused by lower in-cylinder temperatures. EGR decreases ce{NOX} emissions due to lower in-cylinder temperaute; however, ppmV are still higher than minimum regulations.  
midrule
mytab{Saravanan et~al. [cite]}
& Experim. &
mytab{Hydr.-Diesel PFI} & 
Impact of urea SCR on emissions &
Reductions of up to 74% in ce{NOX} emissions were possible. However, ammonia slip into exhaust was a fundamental issue as well as the operating life of the catalyst.  
midrule
mytab{Kamil and  Rahman  {[}cite]}
& Experim. &
mytab{Hydr.-Gasoline PFI} & 
Impact of Hydrogen addition on gasoline PFI SI engines &
Small hydrogen addition of up to 10% favourably increased LFS; however, similar to PFI Diesel engines BTE was reduced due to lower volumetric efficiency. Higher combustion efficiency was experienced ascribed to higher in-cylinder temperature promoting operation near-ideal otto-cycle  
midrule
mytab{Wang et~al.  {[}cite{]}}
& Experim. &
mytab{Hydr.-Gasoline PFI} & 
Impact of equivalence ratio on hydrogen-gasoline combustion dual fuel SI engines &
Lean fuel combustion at an equivalence ratio of 0.7 attained stable and efficient combustion due to hydrogen's fast LFS and flammability ratios. COV was found to be lower up to 10% hydrogen addition since combustion duration is shorter  
midrule
mytab{Ji and Wang  {[}cite]}
& Numerical &
mytab{Hydr.-Gasoline PFI} & 
Impact of equivalence ratio on dual fuel SI engine emisssions &
Carbon monoxide emissions decrease fundamentally from stoichiometric to lean since more oxygen is available at lean conditions to oxides ce{CO} to ce{CO2}.  ce{CO2} emission also decreases with lean operation due to lower specific fuel consumption. ce{NOX} emissions increase to an equivalence ratio of 0.85 and decrease beyond that value  
midrule
mytab{Yu et~al.  {[}cite]}
& Experim. &
mytab{Hydr.-Gasoline DI} & 
Impact of hydrogen direct injection strategy on characteristics of lean burn hydrogen–gasoline engines &
Hydrogen injection pressure has more impact than injection timing. With 4MPa and 5MPa having the highest IMEP since they form hydrogen distribution that is more easily ignited. Also, late injection timing lowers IMEPas it allows for a more homogenous mixture that does not form a stable flame kernel  
midrule
mytab{Yu et~al.  {[}cite]}
& Numerical &
mytab{Hydr.-Gasoline DI} & 
Impact of hydrogen direct injection on hydrogen mixture distribution, combustion and emissions of a gasolineslash hydrogen SI engine under lean burn condition &
Validated numerically the results of Yu et~al. using a CFD software. However, since injector was located near intake port, early injection resulted in higher in-cylinder pressure and efficiency due to more concentrated mixture near the spark plug. Emissions also decrease with injection timing due to more homogenous mixture allowing for complete combustion  
midrule
mytab{Elsemary  et~al. [cite]}
& Experim. &
mytab{Hydr.-Gasoline DI} & 
Impact of SIT on combustion, efficiency and emissions &
Advancing SIT towards TDC realised lower fuel consumption and higher BTE since more work is done during the power stroke. However, carbon monoxide emissions increase with late SIT caused by lower combustion durations increasing incomplete combustion.  
midrule
mytab{Duan et~al.  {[}cite]}
& Numerical &
mytab{Hydrogen PFI SI} & 
ce{NOX} missions in a Hydrogen Fuelled  PFI SI Engine &
ce{NO} constitutes 97% of all ce{NOX} emissions due to increased temperatures, ce{NO} further decomposes into ce{NO2} after combustion when temperatures cool down. Furthermore, thermal ce{NO} was most prominent at 70% due to in-cylinder temperatures while prompt ce{NO} was limited due to lean-fuel operation  

end{xltabular}
end{document}