edit /usr/share/themes/YOURTHEME/gtk-2.0/gtkrc

Look for the sections called "Font Definitions" and "new fonts for fremantle". Just bump up the font size a little in those two sections, save the file and reboot. I'm using the default fonts and increased the size by 2 for all fonts. It is an improvement in readability, but you will notice a few spots where the fonts are a little too large for their particular UI element (FMTX widget, the date on some menu items), but it's worth it.