I’m far from an expert on anything bike-related. I have figured out how frame size works for me though.
Excuse my crude drawing below.
This is a representation of my Raleigh Record single-speed bike. I feel that it fits me the best. I measured three tubes (top tube, seat tube, down tube). Each measurement is given in centimeters. That bike was advertised as a 59cm.
My Surly LHT is almost the same size. I bought the Surly thinking it was a 56cm, but I had test ridden it, and it seemed to fit. Based upon geometry numbers from Surly’s Site, I’m now convinced it’s a 58cm frame. It does have a longer top tube, which explains why I have a shorter stem on the bike.
There’s a lot more to bike geometry than these three measurements, but these three (plus standover, which varies on tire size) are the most important for proper fit.
I figure having the information here will be handy later when doing research for another bike… a bike that will be built from the frame up, so I won’t be able to test ride it.