Set bigger fonts in Qt5 applications

In terminal fire up qt5ct and choose second tab, Fonts, and choose the desired font and size. Further fine tuning by choosing create font.conf will let you make a font.conf file in
/home/username/.config/fontconfig/  and you can set anti-aliasing, and more. You can  increase the dpi value to 128 to increase letter size for example.
To check the dots per inch of your screen use:

xdpyinfo | grep -B 2 resolution

and check out the line resolution.

Optimize for bigger fonts in Firefox

First set zoom to  120% in Settings Zoom. Use Fixed Zoom add on for easy access.
Set fonts in Preferences, fonts and colours. Don't forget to adjust minimal font size in Advanced and untick  "Allow pages to choose their own fonts, instead of your selections above".
Make a Chrome folder in your home/username/.mozilla folder. Download template here and paste in userContent.css:
* {
  line-height: 2em !important;
}
html, body, div, section, article, p, span, th, td {
  line-height: 2em !important;
}
This helps to prevent problems with line spacing.

Set bigger letter size for my fonts in X

My eyes are not getting better with my growing age.
Using a 24-inch monitor I can set big letter sizes for my fonts.
Added to .Xresources in /home/username/
Xcursor.size: 48
Xft.dpi: 120
Reload with:
xrdb ~/.Xresources

Next to that in /home/username/.config/gtk.-3.0/ settings.ini
gtk-font-name=Avenir LT 65 Medium, 22
gtk-cursor-theme-size=48