I have the misfortune of being in between sizes for Cannondale race bikes. Technically I should probably be on a 52, but I have some back issues that make them really uncomfortable for me. So I ride a 54 without much seat post showing.
One of the things to keep in mind about running a larger frame is the crank arms. My CAAD 10 had 172.5, and my Evo had 175mm arms; going to the right size arms for me at 170 really made a difference.