Some folks are having issues with modifying the file correctly, I took the liberty of putting Kenze's notes into a readme.txt along with both the original and modified versions of the Fonts.xml file.
I added +4 to size="#" number for the PetitaMedium and futura fonts sections in \Data\Gui\Customized\Fonts.xml file.
After poking around in game, the only font I've seen with a noticable size difference is the chat window and its tab button and menus. Other fonts like in in Journal, Lore, Achievemens and Tooltips have no noticeable improvement.
Please let Kenze know if there are any additional questions for concerns. Feel free to ask me, I'll help if I can.