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.