Go to your python folder\Lib\site-packages\easygui and then open the easygui.py file in your text editor. If you scroll through that file it's very close to the top. You should see this in the code and you can just change it accordingly.
PROPORTIONAL_FONT_FAMILY = ("MS", "Sans", "Serif")
MONOSPACE_FONT_FAMILY = ("Courier")
PROPORTIONAL_FONT_SIZE = 10
MONOSPACE_FONT_SIZE = 9
TEXT_ENTRY_FONT_SIZE = 12