I've been wanting to know this info for the longest time. And I am constantly trolling X for answers, but to no avail. I do believe the max width is approaching 2¾" wide. I'm talking a handful of examples in the world. I have surmised this from comparison photos, not hard evidence.
I also think the more "common" monsters are between 2¼" and 2½" wide. I have no idea what an average width is, but even an honest 2" wide can look crazy depending on the girl and the guys body.
If you take girthmaster, for example... His width in photos doesn't even come close to the crazy wide look of Chris Diamond. It just doesn't have the same visual impact at all.
I do believe girthmaster is in the extreme girth category however. It's just due to the fact he is more "tubular" that it doesn't look as impressive as Diamond's which I guess is a tad over 2¼" wide at the mid girth hump.
Thanks for bringing light to this "width" over "girth" issue, and I hope we can nail down some facts on the matter. (With caliper measures... For science!

)
Roxx.