I use 2 3/4" #8 on Valley Quail, and typically use 2 3/4" #6 on Huns and Chukar which I find together more often than not. Those same #6 loads work for early season pheasants that are holding for the pointing dogs.
If I am hunting on the prairies and the birds (Sharpies and Huns) are holding, I use 2 3/4" #7.5. If they are getting up at the edge of range, I move up to #6.
When pheasants get spooky, that is what a few boxes of #5 shot are for. I haven't seen much of a difference between 2 3/4" and 3" loads but if you get a deal on them, then I'd just grab 3" as more shot in the air probably can't hurt anything other than your shoulder.