it all depends on how fast you can spin comfortably. also, hills, drops, etc.

i started out 48x15, which was rough for a first timer in my case. i am comfortable right now with 48x17. Davis is not too hilly, but when i travel to my parents place, i can get up the hills and down them pretty well.

easiest thing to do is try a few cogs (they're cheaper than multiple chainrings). find what's good for you.