Fast - that is 19 5/8" Dia. It just looks big through your divers mask in that cold SF Bay water. Back here in the thick water of the Chesapeake, it's just right.
Oh. When we (hull cleaners) talk about Max Props, the reference we use is hub diameter. Most are 63mm, 70mm or 83mm. A really big one would be 100mm. I know they make 'em bigger than you could lift, though. Thought you had one.