I ride a 56 from one bike company and a 58 from another. You can't necessarily compare one brands size to another. I got burned by this once when I bought a used bike on Ebay. Buying without riding it is to some degree going to be a guess. Therin lies the problem with buying online.

Of course, just getting the frame size correct is only the first step in a good bike fit. Then there are all the other variables. A good LBS would, of course, help you with this and change out parts (usually for free) that need to be changed to get a correct fit.