If you can comfortably ride a 50cm Cross Check (they have tweaked the geometry recently, and they're even longer in the TT than they were in the past), I would say that the Colnago is going to be on the small side.
Torso length-wise, we're probably similar. My CX bike has a 53cm ETT, though on a Cross Check I'd be more comfortable on a 46cm. I'd feel really cramped on something with a 51cm ETT. You could put a longer stem on it, but my guess is that it already comes with a 90-100mm stem, and I wouldn't go beyond 110mm because of how it affects handling.