It really depends on your body. I am 6' and ride a 56 frame because I have short legs in relation to my torso. By doing that, as opposed to the next larger size, I am able to get plenty of saddle-to-bar drop, but still be comfortable. When I ride larger bikes, it feels like a hybrid because the headtubes are usually so freakin' long.

Ride what you feel comfortable and fast (or whatever your goal is) on.