Just looking at pictures it appears to be 3" or 4".
While I can't speak for Bludawg and others participating in this discussion, I am under the impression that nobody is interested in building an exact copy of the PBC. I suspect that they, and certainly me, are more interested in discussing what makes the pbc "tick", if you will.
The first step in any build is research and planning, right? Thats all we're doing. Personally, if my intent was an exact copy, then I WOULD buy one, and copy it. PBCs are built the way they are because the owner thought that's the way they should built. As a person who has been building things all my life, I've got my own ideas on how it should built. And that's how I will build it, MY way.
So at that point the PBCs are nothing more than an inspiration, not a blueprint.
UDS, mini wsm, 22.5" weber kettle