The size of a frame, in general, is the distance between the center of the bottom bracket (center of the cranks) to the top of the seat tube. As Sheldon Brown points out, it's not all that simple, really.

Your inseam (distance from floor to crotch) is the single most important measurement for fitting, but by no means is it the only important measurement. Second on the list is the "cockpit," or the distance from the seat to handlebars. The cockpit has to match your torso, or you'll feel very uncomfortable on the bike.

The bottom line is that even if you get fit for a Trek 56 cm, for example, it doesn't mean squat about how you'll fit on other manufacturer's 56 cm bikes, or even other Trek models. You really need to ride a bike to know if it fits you. There really is no better way. Once you're experienced and have gone through a few bikes, you might be able to look at a frame's geometry table and know at a glance if it'll fit you or not, but, for your first bike, you really need to go test ride as many bikes as you can get your hot little hands on.

- Warren