I am 5' 6" exactly. I ride a 51 cm Lemond Maillot Jaune, a 51 cm IF Crown Jewel, a 51 cm Scattante R-650, and a 50 cm Colnago Super. All these measurements are ctc except for the Scattante which measures ctt of the seatpost clamp. I find that the Lemond and the IF are a bit long in the top tube for me, so I had to drop the stems to 90mm and go to Thompson (no setback) seatposts. The Scattante and Colnago fit just about perfectly. Of course, proper sizing will depend on the relative lengths of your legs, torso, and arms as well as just your height. I have relatively long legs for my height, so bikes that fit me in the seatpost tend to be too long in the top tube.