What is the largest sized rocks you can tumble in a 6 pound barrel. Can you use one 12 pound barrel on this?

Yes, you can use one 12 lb barrel on this model as well. As far as the largest rock size, this is more of an advanced technique. If you are new we'd recommend you start with smaller sized stones. Under the correct conditions, with proper fill level and slurry, the size of the stone could be around fist size, maybe bigger. But you will have to do many things correct, in order to get this to work.