Here's a comparison of some geometry numbers from 2009-2010 bike frames. I plot head tube length versus reach.
Now it's a standing question in the age of sloping top tubes what's the single best number to describe the size of the frame. The only parameters which can't be adjusted are front-center and rear-center and bottom bracket drop, so these are likely candidates. Considering the body fixed with relation to the bottom bracket by an appropriate choice of seat post length and offset, stem length and angle, the front center and chainstay length, in combination with the bottom bracket drop, determine how weight is distributed between the front and rear wheels. But it doesn't feel right to use these parameters to describe bike size. After all, a touring bike designed for a short person might have longer dimensions than the longest Cervelo road racing machine, the Cervelos known for their tight wheelbases.
The next candidate is Cervelo's favorite number: reach. This describes the lateral distance from saddle to handlebars with seatpost chosen to yield a fixed saddle setback with respect to the bottom bracket. Of course a fixed stem assumption is unrealistic, but generally one want to keep stem length within a certain range (8 cm for a short rider up to 13 cm for a tall rider) to keep a certain range of handlebar motion with steering. So for these plots I'll use reach.
The other parameter is head tube length. This is the other geometry parameter describing where the handlebars end up. I figure you generally want a head tube which requires neither a tall stack of spacers nor one which requires slamming a -17 degree stem against the end of range. You want something which puts a stem reasonably close to the head tube while allowing for some range.
I'm sticking with carbon frames in this analysis, since they tend to have internal headsets, while there's still a lot of metal frames out there with external headsets. External headsets add to the effective head tube length.
Okay, the data... click on images to view larger versions:
It's clear there's a wide range of geometries available, despite the seeming homogeneity associated with the dominance of OEM Taiwanese manufacturing (Trek and Time being exceptions). Despite this, on the WeightWeenies forum you get people all the time asking questions like "Should I get bike X or bike Y?" without providing any information about their body size or geometry preferences. It's almost like asking which shoe should I get, Nike or New Balance? Well, which one fits better?
True, pro cyclists tend to make do with the geometry of their sponsoring bike company. But there's no reason for you or me to live with that restriction.
What about me? I think I like the Madone Pro Fit geometry the best: ideally I'd like an 11.5 cm head tube @ 72 degrees, a 75 degree seat tube, and a 39 cm reach. None of the bike nail this head-on, but a bunch of them are in the neighborhood. But not all, obviously.