FYI, both the ARCA-SWISS B1G and B2 were discontinued several years ago. They are occasionally available on the used market, but neither are currently available new.

Photo Clam offers an extra large ballhead, the PC-74NS (74mm ball diameter), that is similar in size to the discontinued B1G. Anyone looking for a Really Big ballhead, may want to give it a look. It is indeed massive, and combined with an equally massive tripod, should hold just about anything you'd care to mount on it. You can see the specs on it here:

There is a user review here with several photos of this ballhead with a 600mm f4 super telephoto lens that weighs 5.36kg/11.8 lbs. (warning - digital content alert!):

Kerry Thalmann
Really Big Cameras