In addition to what Kontact said above, I would add that your *proportions* need to be taken into account when sizing a bike. I'm 5'9", and I was told over and over that a 54 was the right size for me, but it wasn't. I have very long legs for my height, so actually I needed a top tube 1 size smaller--52. (With a 100mm stem, rather than the 90 that the bike came stock with. On the 54 bike I first bought, I had to go down to a 80mm stem, which made the bike very twitchy.) when I went to buy my second bike, again, the salesman was trying to put me on a 54, but as soon as I got on a 52, I knew it was right!

So I recommend you measure your cycling inseam carefully. If you have short legs, you might need a longer top tube, long legs, a shorter top tube. Go try a bunch of bikes. If you can try a model in the different sizes, so much the better.

How tall are you, and what is your cycling inseam?