Quarry uses the GTK+ theme application font, so when you change the font size for your GTK+ desktop apps, Quarry should follow suit.
Now the bad news: it might not work in Gnome 3 - I don't even know where to set the font there. But at the time this bug was submitted (and still if using Gnome/GTK+ 2), changing the GTK+ setting was 100% the correct solution. It should be (or have been, at least) under Gnome Control Center / Appearance. Hope this helps.
