I'm on Ubuntu 20.04 and am running the standard gnome.
I've had a look at Change font size of top bar in Ubuntu 20.04 and Install Adwaita shell theme on Ubuntu 20.04? but these are not helping and, in fact, crashed my computer when I edited the resource file.
I also found: https://www.debugpoint.com/2020/04/3-ways-to-increase-desktop-text-size-in-ubuntu-20-04/ which didn't seem to help.
Suddenly my Top Bar, Settings Menu, and login screen has HUGE fonts. Example
Anyway, it makes my eyes hurt.
If I go into settings/universal access, Large Text is off. If I turn it on, it gets smaller. If I turn it off, it gets smaller still. Next reboot, though, resets this back to the big ugly font. If I leave it on and reboot, same thing.
Don't know what to do next. I figure this came in with an update, but can't be sure.