I'm looking for a frame for a road bike so I can customize it with decent parts, and have been doing online research about sizing. I've come up with a broad range of numbers that seem to fall into two groups: a smaller size and a larger size.

My height is 5-9 and my inseam is 31 in.

Here's what I've found (from center of crank to top of seat tube):

About.com -- 56cm
Evanscycles -- 56cm
Bikesizing.org -- 53cm
eBicycles -- 54cm
Sierra Trading -- 53cm
Kleinbikes.com -- 57cm

So is that a 21in bike?