Mostly use a 20 ga and am fine with 5s or 6s lead. Generally an ounce in either 20 or 16 and 1 1/8 if I break out the 12. I tend to prefer lower velocity rounds for better patterns.
2 3/4 5 or 6 what ever is cheap alot of times it will happen to be what ever is in the center counsel while out driving around and seeing a chicken in the ditch
Winchester Super X 1-1/8 oz #6 out of a 16 gauge is pretty much all I shoot. If I'm on an area that requires non tox I'll use Boss #5s. If I miss them with either of those, a bigger pay load wouldn't change anything.
I hunt with a pointing dog, so expect all my shots to be pretty close. I use a 20 gauge and 4, 5, 6, or 7 depending on what I have on hand. Some public areas require steel or have max shot size requirements, so adjust accordingly. Only thing I really try to avoid is 7 and smaller in steel for pheasant.
Late season or real wild flushing birds...Federal #4 or #5 Prairie Storm hands down. ZERO reason to go to a lead or BB. Grew up upland hunting with a Grandfather that owned a Pheasant hunting lodge in S.D. I also used to guide at a different operation wild and released birds. 90%+ of the time I shot regular premium lead 6s or maybe 5's. If you aren't killing birds you aren't hitting them and pellet size has nothing to do with it.
I don't even shoot BB's for ducks and geese. TSS is just more expense. Bismuth IMO is overrated for the price.
I shot 39 roosters this fall between SD and MN. Many of the birds I shoot are as a poster/blocker or off to the side of a friend or family member working a dog. (don't currently have my own dog). Until I get my own dog and start getting close opportunities at birds flushing off of a point, I have zero reason to shoot 6's. I want heavy payload high velocity copper plated 5's and 4's all day. 2 3/4" 1 3/8 oz or 3" 1 5/8 oz. in 12 gauge or 3" 1 1/4 oz in 20 gauge. High velocity 4's might make a little bigger hole in the meat, but I rarely find pellets in the breast meat, as they get better penetration. Of the 5 birds I cleaned from saturday & sunday, I only popped one pellet out of a bird and that was in a thigh.