A 9" diameter Dynamax is the "official" Hooverball. Either 4 or 6lbs. By "official", I mean the one that is used in the Hooverball National Championships in West Branch, Iowa.
I would not want to play Hooverball with any ball that is not padded. Good players can really wing that thing at high velocities and it would be downright dangerous with a hard ball.
Here's some pics of the official ball.
If you've never played with both a 4lb and 6lb ball, I highly recommend trying them both. The games are significantly different. Ironically, I think the bigger, stronger guys are better with the 4lb ball because they can sling it so hard.
The 6lb ball has enough weight that you can't sling it so easily and the smaller, more mobile guys can zip around to catch it easier. It's fun to play both.