Originally Posted by Charles Wahl

While it doesn't solve all (or even any) of the problems, I have always liked the piece that Sheldon Brown wrote about frame sizing: Revisionist Theory of Bicycle Sizing . He quite reasonably points out that in an age of "proportional sizing", confusingly multiple "standard" references for measuring "seat tube height" sloped top tubes and long seat posts, measuring frames based on seat tube is fairly pointless, and a much more important feature of a bike's sizing is the top tube length, considered concurrently with the variables available: saddle/seat post fore and aft adjustment and stem length (offset between steerer tube and handlebar mounting). The seat post length is highly and easily variable (unless one is constrained by limitations imposed by vintage components or style) while the range of movement available through saddle or stem adjustment/modifications is smaller. He also points out that the type of cycling one does (or is capable of, or aspires to) affects the "correct" relationships of saddle and bar locations and the cranks and wheels. In the end, all this is a moving target and some people wander and wonder about it chronically, while others seem to find their happy place and seem to replicate that (with individual adjustments) on each bike they ride.