I just reworked the bracket on my car, I wish I had measured the ball. I do know that the bushing OD is 1" diameter. There is not much "meat" in the thin part of the bushing, so I would GUESS 3/4" on the ball. Probably not much help for you, though.
While you didn't ask, the best replacement bushing is available from VanPelt Sales. Their version is durable nylon that would seem to be an improvement over the more easily fractured metal replacements.