Increase the width of the arrowhead and shape boundary

• Your flowchart is based on an old example which use now deprecated syntax for defining flowchart elements syntax (tikzstyle instead of tikzset, without library arrows.meta).
• Library arrows.meta enable customization of arrows heads (length, width, angle,
  fill, scale, etc). It is described in TikZ & PGF manual, section 16.3 Arrow Keys: Configuring the Appearance of a Single Arrow Tip, pp. 193 -- 206 (version 3.1.7a).
• Shape border thickness can be increased by adding desired border width. In this you can use predefined widths: semithick, thick, very thick or ultra thick or define it by line width=<desired width>. In MWE below is selected semithick for arrows and thick` for nodes borders.
• With use of the libraries positioning is possible to write more clean and shorter code without any manual tweek of position with xshift and yshit options.
• Further simplification of code can be achieved by use of the chains library. Using its macro join can replace drawings most of the connections between nodes.

Considering aforementioned, The MWE of your flowchart can be:

documentclass[12pt,english]{article} % 14 pt not exist
usepackage{nopageno}
usepackage{blindtext}
usepackage[paperheight=13.1in,paperwidth=7.9in,margin=0.05in,heightrounded]{geometry}
usepackage{tikz}
usetikzlibrary{arrows.meta,
                chains,
                quotes,
                positioning,
                shapes.geometric}
tikzset{
        arr/.style = {-{Stealth[length=7pt]}, semithick},   % <--- defined arrows heads style
      base/.style = {draw, thick, % <---
                    text width=42mm, minimum height=8mm,
                     font=large, align=center},
 startstop/.style = {base, rounded corners},    %, fill=red!30
   process/.style = {base, rectangle},          %, fill=orange!30
        io/.style = {base, trapezium, trapezium stretches body,
                     trapezium left angle=70, trapezium right angle=110,
                     },                         %fill=blue!30
  decision/.style = {base, diamond, aspect=1.6, %fill=green!30,
                     inner xsep=-3pt},
        }

begin{document}
centering
    begin{tikzpicture}[
node distance=6mm and 8mm,
  start chain = going below
                        ]
% main branch
    begin{scope}[nodes={on chain, join=by arr}]
node (start)   [startstop] {Start};
node (pro1)    [process]   {Make};
node (pro2)    [process]   {Initialize};
node (pro3)    [process]   {Increment};
node (dec1)    [decision]  {satisfied?};
node (dec2)    [decision]  {satisfied?};
node (dec3)    [decision]  {Achieved?};
node (pro4)    [process]   {Store};
node (dec4)    [decision]  {Maximum reached?};
node (pro5)    [process]   {Select model with max};
node (stop)    [startstop] {Stop};
    end{scope}
% left branch
node (dec2a)   [decision, 
                 left=of dec1] {Thickness within maximum limit?};
node (pro3a)   [process,
                 at={(dec2a |- dec2)}]   {Increase};
% labels in main branch
path (dec1)    to ["Y"]    (dec2) 
      (dec2)    to ["Y"]    (dec3); 
% arrows and labels right from main branch
coordinate[right=of pro1 -| dec4.east] (aux1);
draw [arr] (dec1) -- node[pos=0.25, above] {N} (dec1 -| aux1);
draw [arr] (dec2) -- node[pos=0.25, above] {N} (dec2 -| aux1);
draw [arr] (dec3) -- node[pos=0.25, above] {N} (dec3 -| aux1);
draw [arr] (dec4) -| node[pos=0.25, above] {N} (aux1) 
                   -- (pro1);
% arrows and labels left from main branch
draw [arr] (dec2a.west) --  node[above] {N} ++(-8mm,0)  |- (pro1);
draw [arr] (dec2a) to ["Y"] (dec1);
draw [arr] (pro3a) -- (dec2a);
draw [arr] (dec3)  -| node[pos=0.1, above] {Y} (pro3a);
    end{tikzpicture}
end{document}