I just impulse-bought a Soma Saga disc frame and fork, as they were having a 20% off sale ($500), and I have enough spare parts at this point to build it up. Based on my custom-fit frame, the geometry was closest to a size 52, but size 54 is the smallest that permits 700C rather than 26" wheels. The following summarizes a subset of measurements for comparison.
Code:
Custom: Saga (54): Saga (52):
Reach: 371 mm 381 mm 372 mm
Stack: 577 mm 580 mm 567 mm
Eff Top Tube: 543 mm 555 mm 540 mm
Seat Tube: 497 mm 515 mm 495 mm
Stem: 110 mm
Saddle center
to Stem/bar
interface
center: 652 mm
Briefly, the 54 reach is a bit (maybe 10 mm) too long, but has about the right stack, and allows me to use my extra 700C wheelset. Since my custom bike has such a long stem, I assume going a wee bit shorter would allow me to keep the same saddle center to bar length. I also remember that my 1990 Trek 520 (a size 54) fit really well and had similar proportions. (I gave it away to one of my grad students about 12 years ago.)
There is probably time to change my order if this is a bad idea. Since the frame costs less than the White Industries crank I am going to put on it, and I am mainly going to use this for tours and hauling large amounts of stuff locally, it isn't the end of the world if it isn't absolutely perfect. Also I have 2 kids who are slightly taller than me now, so I could always give it to one of them.
What say you?
Also, since the reach is slightly longer, it has me thinking of non-conventional bars. I have trekking bars, Jones bars, and various mountain bike bars sitting around unused at the moment...