Steve sends this piece from Sheldon Brown at Harris Cyclery
Top Tube Length: More Important Than Seat Tube Length!
When people speak of bicycle frame sizes, they generally speak in terms of the seat tube length. As mentioned above, this used to be the only variable, but with proportional sizing it no longer is. I would submit that seat tube height is no longer the most important frame dimension. More determinant of the actual way the rider will sit on the bike is the top tube length. It is obvious why you shouldn’t have a bike that is too tall to stand over with a reasonable safety margin (although even this sizing practice was not universally accepted for the first 30 or 40 years of the diamond frame.)
On the other hand, why shouldn’t you ride a “too small” bike? “Because the seat and handlebars will be too low!” That was a good objection ten years ago, when tall seatposts were a rarity and quality handlebar stems were available in a variety of forward extensions but only one (short) height. All that was before the mass production of the mountain bike. Now 250 mm and 300 mm seatposts are stock items, and a variety of excellent handlebar stems are available
There are a number of sizing systems available today, which require various measurements of the cyclists body and reccommend frame sizes on this basis. Probably the best known of these is the New England Cycling Academy FitKit. I use this system myself, but not in a blind, rote manner.
The FitKit makes recommendations for a particular seat tube length, and a range of top tube lengths with corresponding handlebar stem extensions. For instance, for a particular rider, it might suggest a 58 cm seat tube with a combined top tube and stem extension of 66 cm. This 66 cm might be from a 61 cm top tube with a 5 cm stem, or a 54 cm top tube with a 12 cm stem, or any other combination that adds up to 66 cm.
Any of these combinations will give an equivalent posture on the bike. One or two combinations are particularly recommended because the more extreme variations of stem length can cause a bike to handle strangely because of the positions of the hands relative to the steering axis. The “by-the-book” fitting method would then be to select a suitable bike with a 58 cm seat tube, measure the top tube, and install the recommended stem.
I would submit that this approach is due to the old fashioned fixation on seat tube heights. Better, in my opinion, to find a bike with the ideal length top tube, fit the recommended stem, and not worry about the seat tube size, within reasonable limits.