Outdoor life magazine came up with this very handy chart. It's not only important to choose the right size shell (2 3/4, 3'', 3 1/2'') but also the right size shot.
It's also important to shoot and pattern your shotgun with different manufacturers shells. I find that all shotguns shoot a little different and prefer their own ammunition. Always take it to the range first, KNOW it's affective range and shoot within YOUR abilities before going after any game animal.