There are more variables than what you've mentioned. It is also dependent on your height, other body measurements, the rise of the stem, how many spacers are underneath, what your planned use of the bike is, and you get the picture. For example, on my 56 cm crosshairs, I have a 90 degree 100mm stem, I'm 5'10", with a 32" inseam. On my 54cm crosshairs, I run a -10 degree, 100mm stem. I use the 56 for riding around, wet weather road bike type stuff, and I race the 54 cm. Your best bet is to find a fit professional to size it up.