Originally Posted by Mike Mills

You need to know the diameter and threading of the cups/retainers that thread into the bottom bracket.

A more difficult part is getting the proper spindle. The spindle width is dependant upon bottom bracket width but also upon the chainline needed. This is dependant upon the crank you are using (single, double, triple chainwheels) and the rear axle spacing. You also need to know the taper style on your crank (JIS, ISO).