Is there a specific animal you're not sure about? Here's a few common ones and the ideal shot (IMHO) if you are only going after that animal. If the possibility for several species exists, go with the middle or larger shot. General rule for steel is use 1-2 sizes larger.

Dove, quail 7-8
Rabbits, squirrel 6-7
Pheasants 4-6

I don't do much waterfowling, someone else would be able to give you recommended shot sizes better than I. One note though, I went dove hunting last Sept. on a waterfowl/wildlife refuge, and we had to use steel shot for doves. The borders were well marked with signs letting you know that steel shot was required.

Hope this helps,

Lance