Resize line numbering

I use Texmaker to help with writing in LaTex. I have added line-numbering to the draft of my thesis. The size of the line number is a bit too small. Is there a way to make the size a bit bigger?
The preamble and some of the code is:

documentclass[a4paper, oneside]{book}
usepackage[left=35mm, right=15mm, top=20mm, bottom=15mm]{geometry}
usepackage{setspace}
usepackage{fixltx2e}
usepackage{graphicx}     %For adding Figures
usepackage{float}        %For removing figure borders
usepackage{tabularx}      %For making tables
usepackage{amssymb}
usepackage{booktabs}      % For toprule, midrule and bottomrule
usepackage{pgfplotstable} % Generates table from .csv
usepackage{longtable}     %Allows multi-page tables
usepackage{rotating}      %Allows for single page landscape using 
usepackage{pdflscape}     %Allows for landscape
usepackage{multirow}      %Allows for merging across rows
usepackage{tablefootnote} 
usepackage{url}
usepackage{footnote}
usepackage{lineno}
usepackage{threeparttable}
usepackage{caption}
captionsetup{singlelinecheck=false}
usepackage{siunitx}
sisetup{input-uncertainty-signs=pm, 
        separate-uncertainty=true}


begin{document}

begin{titlepage}

Insert title page content here

end{titlepage}

tableofcontents
newpage
mainmatter
linenumbers

chapter{Introduction}
setstretch{1.5}
fontsize{14}{18}selectfont

This portion contains the general body of work.
It contains a lot of text, numbers and citations.
There are plenty of pretty figures and LOTS of tables.

end{document}