TextEdit keeps changing font size from 12 to 14 when I click back into the window

If you create another fresh user account on your Mac, does the problem follow?

(You can delete the account after you've tried it out.)

If rebooting your Mac doesn’t fix things, see if the problem still happens in Safe mode (hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's normal.

You won't want to conduct any business in safe mode, but sometime just booting this way, logging into your account and then restarting a few minutes late can clear the cobwebs.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

If the problem is still unresolved, please return here and post a reply and we’ll see what else we can come up with for help.