Does your BB size conform to the size to the shell, or what? When I look at cranks they tell me what size BB to purchase with the crank...so where does the shell come into play?
Thanks, I read the section on BB and cranks in Zinn's maintaince manual at midnight last night and I have to much homework to read it again.

You need a few critical dimensions:
1) Cup thread and diameter, generally 24 TPI and 1.37" English/ISO on newer bikes, although I THINK some (Italian) frames are still being made with 24 TPI x 36mm Italian threading, for some inexplicable reason.
2) Bottom bracket shell width, generally 68mm (English/ISO road, also obsolete French or Swiss), 70mm (Italian road, generally with Italian threading), or 73mm (English/ISO for fat-tubed frames).
3) Overall spindle length, a function of number of chainrings and brand of cranks, typically something like 102 to 123mm.
4) On old-school non-cartridge BBs, the thickness of the cups ("thick" or "thin") and the spacing between the bearing races on the spindle.