You could open the gtk file for that theme and 'comment out' the lines that specify a font size if there are some. Copy it into a folder or onto your desktop first so you can put it back how it was if you go wrong, typo or whatever.
I don't know if this is the standard method, but I would do this by holding down alt, pressing F2, typing
in the box, enter your password, and then navigate to usr > share > themes > (then the name of the theme you use, mint-x was it?), open the gtkrc file in Gedit text editor and put a # in front of the lines that start
in the same way that other lines have that # in front. Save that then and either change the theme and back or reboot. That should do it, I'm no expert but I've done 'worse' with no ill effects...