If your looking for plinking and squirrel, then a .32 or .36 is normally what is used. But as Semisane said, I was hunting them with a .50 caliber loaded light, using roundball. And I would only shoot if they had tree behind them. It worked just fine and its easy to bark them too. The problem with loading a hot load (for me) and then shooting up into a tree.. if you miss, where is that ball coming down.
Years ago, there was an incident where a person discharged their muzzleloader and it killed an Amish woman driving a buggy over a mile away, when the load came down. Freak of nature, terrible accident, of course it was. But it did happen. So pay attention to what's behind your target when you shoot.
The idea of a smoothbore shooting shot is a great idea. You can load them light, you miss less, and its an easy way to make hunting a lot of fun. Plus with a smoothbore, you can hunt big game with them if you so wish. Simply load a round ball. Your range is limited but not your fire power.