You know what? I rethought this.
I'm good with any diameter barrel that comes on the stick. If it's at least 18 inches (but preferably a bit longer), I'm good with that too.
The thing is, I can pop the barrel out in a heartbeat (thanks NY, for making me think) and send it off to be contoured if it's too thick for my tastes. In the same vein, I can have it shortened if it's too long - the only thing I can't do is add length to it. Therefore I'd prefer it to be no shorter than 18 inches. 20 or 22 is ideal for me.