Sir Samual Baker used 12 drams in his 2 bore. (about 325 grains)
I would start with about 100 grains and work up 25 grains at a time, until you get to what you can't handle, OR until you get to 325 like Sir Sam did.
At 325 grains, I believe you should be able to kill anything on dry land that doesn't go "clankity clank" when it moves, and have tracks under it.