It's not clear whether you are speaking about one label or a whole sheet, but I'll assume the latter. Once you have them set up, for mail merge or otherwise, simply select all (Ctrl+A), and increase the font size. you can increase font size one point at a time with Ctrl+] (use [ to decrease) or by larger increments with Ctrl+Shift+> (< for decrease). Actually, those are Windows shortcuts; sorry, I didn't notice you're a Mac'y. Mac undoubtedly has similar ones.

Does that do the trick for you?