There's a pretty solid guideline on thru-hiker.com's site, that's been the baseline for a while, and from my experience, it's about as accurate as can be.
Generally speaking, I always target 2" nominal loft for 30deg, and 2.5" loft nominal for 20deg, and usually observe more like 2.2 and 2.75, with the same 900fp down that Joe is using. That being said, I've been accused of making quilts too warm by many users, and considering the enclosed nature of these bags, they may be more efficient, I couldn't say.
I would definitely ask Joe for the actual "usable" flat widths or circumferences of the sections of bag you're interested in knowing the "girth" measurements for, since "girth" has become an ethereal and never consistent spec.
I do know that the fabric widths of the shell materials he's using aren't wide enough to accommodate those girth specs once linear shrinkage and hems allowances are factored in, without joining to panels horizontally, which doesn't appear to be the case from the pictures.
That being said, girth numbers have often not added up to real world fabric widths, with many mainstream manufacturers, so I'm not sure what the baseline is.
Looks like a nice option for back sleepers from Zpacks, with a *lot* of top end materials, for a very reasonable price.