At first glance this sounds like a duplicate of Can't change text size in gedit, but it isn't: in that question they're actually talking about print size, I'm not.
I'm using Ubuntu 14.04 and Gedit 3.10.4.
Anyway - last week, to let one of my kids play with the text editor, I changed the default text size to be large, on the Edit/Preferences/"Font and Colour" tab. Now, I can't change it back - I tick on the box to use the default "Ubuntu Monospace 13" font, and the box is ticked and then unticks itself straight away. I can change the font size instead, and the change just reverts as soon as it's saved.
I think it might be due to some config file which has the wrong permissions: if I open Gedit in sudo mode, I can change the font size. But the change made here doesn't cross over back into "non-sudo" gedit.
The only config file I can find in home is
~/.config/gedit/accels and the permissions are normal on that. I think that's just a list of keyboard shortcuts anyway. There's lots of other config files in
/usr/share/gedit/ but none of them show as being modified recently.
I've tried uninstalling and reinstalling gedit but it didn't make any difference.
Any ideas anyone?