Rule of thumb 2 sizes bigger than lead. Works pretty good, or go to the denser non-toxic. I pheasant hunt on private or walk-in where lead is allowed but have bismuth in the truck in case I need non-toxic, can't shoot steel in the O/U. Been carrying the same shells for years. Tom Roster's 2016 Nontoxic Shot Lethality Table

It all depends. Hunted with a friend who liked steel BB for pheasants. 20 ga. no less. They all dropped like a rock. Harry was an excellent shot.

The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh

Which explains a lot.