You cannot customize the fonts and appearance within Gramps itself, but it is quite straighforward to modify the GTK theme settings.
For the latest version for Windows (4.2.1), find the config file. Go to the base path for Gramps in Program Files > etc > gtk-3.0 > settings.ini
Open up the settings.ini and add or change a line:
gtk-font-name = Arial 8
(or whatever font and size you want).
Making this change alone I found the buttons to scale to an appropriate size.
You can also modify other aspects of the theme by browsing the style files in the themes folder.
Keep in mind it would be a good idea to document what you change, because if you update to a later version all these changes will revert, and if you're like me you'll struggle to remember exactly what settings you changed before.