How to change and set font size?

system Preferences > Displays > and look under resolutions. If there are any that says "scaled" try one of those.

What that does is double (or 1.4, or 1.8 ) the size of text, ONLY, while leaving the natural display resolution for graphics unchanged.

If no other settings are showing try holding the option key while clicking on the settings box.

Then there are a whole raft of items under Accessibility options.

font size of WHAT, exactly ?

If you mean the font of the OS user interface, that cannot be changed.

If you mean something else, you need to tell us what exactly.