Behind the Scenes at a U.S. Factory Building New Nuclear Bombs

I think the pit is significantly smaller then a bowling ball, probably closer to a softball or a grapefruit. The diameter at critical mass is 95–99mm.

They make it subcritical by putting a hollow in the center. This hollow was about the size of a golf ball on the fat man design. It has to be large enough so that is does not go critical when immersed in water, but outside of that you want the pit to be as small as possible.

On older designs they filled the hollow with a safety chain made of cadmium balls(like the chain on a ceiling fan) as a Neutron poison. The bombardier would have to pull out the chain to arm the bomb.