Bicycle Mechanics - Measuring threaded headest

I'm looking to build my arsenal of tools and rebuild my headset. I'm working with a Specilized Cirrus from the 80's, so I'm not sure what size I have. What's the best way to actulally measure the size of the headset nut? I can immagine taking two straight edges on opposite ends of the nut, then measuring the distance between these opposite lines. Any other ideas?

digital calipers from the hardware store?

A large adjustable wrench. Most 1" threaded headset nuts are 32mm.

How many are most? I'm looking to order a tool online and I want it to fit.

Get a Vernier. You will need some accuracy with measurements and with a vernier you can measure in and ouside diameters. With a Digital you can do inch and metric.

How many are most? I'm looking to order a tool online and I want it to fit.
All that have wrench flats. There were a few headsets (Shimano only?) that had scalloped lock nuts and took a proprietary wrench but these are obvious.

Park makes a double-ended headset wrench with 32 and 36 mm openings for 1" and 1-1/8" threaded headsets.

I agree with the recommendation to get a vernier or dial caliper. You will need to measure a lot of things as you get into bike mechanics and this is an essential tool.

How many are most? I'm looking to order a tool online and I want it to fit.
What kind of headset? Decent quality, non-French bikes from that era almost always followed the de facto Campy standard of 32mm.

Honestly, I've got 2 headset wrenches, and more often than not I end up using my 12" adjustable, because the headset wrench is a looser fit than I'd like and I don't want to round off the corners of the nut.

Honestly, I've got 2 headset wrenches, and more often than not I end up using my 12" adjustable, because the headset wrench is a looser fit than I'd like and I don't want to round off the corners of the nut.
You need a better headset wrench:

http://os2.dhs.org/~john/stronglight-wrench.jpg