Being a part-time contributer, I've never seen any documents suggesting sizing.
I'm guessing providing information like this falls under the umbrella of services provided by JBoss (Redhat) the corporation. So, it may in fact be intentionally missing.
Anyway, the sizing of things comes down to what is run on top of AS or jBPM and the expected load. It's a bit like asking how big of a machine to run Linux on. It's really what's on Linux that determines the requirements.