Originally Posted by jimmuller

Threaded or threadless? At least for threaded, some key dimensions are the diameter of the crown where the crown race (i.e. the lower inner race) sits, the inner dimensions of the head tube, and the steerer tube diameter. A big unknown is the stack height. Best is to start with something you know, which is to say the spec for your current headset.

See http://sheldonbrown.com/headsets.html