Charles, 
Attached is a photo of the barrel shank of a CPA barrel (32-40) that I measured in machining a new barrel (30-30) last week.
The breech section is .845" diameter and .310" long. The threaded section is .9235" diameter and .815" long and the distal end is .927" diameter and .4" long.
The threads are nominally 15/16"X16 tpi but my action has a smaller major diameter.
If your action has a nicely fitted barrel, I'd recommend that you measure the three areas and turn the diameters a few thousandths larger. 
If you don't have a fitted barrel to measure, the breech diameter needs to fit through your action's threads. Then you can measure the diameter across the threads to get your major diameter and increase the diameter a few thousandths for safety. The final diameter was .005" larger than the major diameter.
Without a fitted barrel to guide you, I'd turn a test shank from brass or aluminum to use as a guide.
Good luck,
Otto   

Otto let out a Closely Guarded Secret.  Turning a test shank has saved my Bacon more than once.  I've found that most Threads on a Barrel are on the loose side.  Good luck 👍🏻. David

CPA got back to me. Interesting they don't dimension the 0.400" section diameter as noted above by chipmaker.