Giant XTC Team Custom XC bike, Nashbar tourer custom commuter
Originally Posted by Svr
The outside diameter of the fork steerer tube is the basis for head tube sizing.
This means that (for example) a 1 1/8" fork will require a larger internal diameter headtube than 1 1/8" to accommodate a headset as well. Exact sizes can vary, so try looking up the model year of your frame to find out what it needs. If nothing else, see if you can have an LBS figure it out for you, if nothing else they probably have headsets for each size that they can try.