Increase font size for old eyes

Old guy here. I want to increase font size so my poor eyes can read the screen. Under custom.el there is a section for font, but it has adjustment for mac and windows, not linux.

I can't go to m-x customize group as it won't accept changes. There is an orange warning of some sort that flashes, but its so quick and so small i can't read it.

https://redd.it/1wubcgs
@r_emacs