RE: shot size for 20 gauge
You have to pattern your gun with different loads to see what works the best in your particular firearm. For example both my son & I hunt Turkeys with 20GA pumps; his a Winchester®, mine a Mossberg®. His gun patterns #5's superbly; mine not worth S#$T! Conversely mine patterns #6's like a house a fire; his not so. Both guns pattern #4's OK but nothing to sneeze at. It's a trial & error method to see what works best in a particular gun.