Originally Posted by
Torchy McFlux
Old fast and dirty bike shop sizing method: Straddle the toptube of the road bike with both your feet flat on the floor. If you've got about an inch of clearance between your crotch and the tube, it's your size.
This is ONLY partially valid with level top tube bikes, which you hardly see anymore.
The real critical measurement is top tube/stem combo. That is what determines weight distribution and to a large degree, comfort.
If you are in a shop and someone has you stand over a top tube and proclaims "it fits" without getting at least a look at you ON the bike, run! You are dealing with idiots.