For grouse typical sizes I hear used are 6, 7-1/2, 8 & 9.
For early season, I find regular 1 oz. target loads of 8 & 9 work very well, especially 9's, when woodcock are part of the equation. Grouse are not hard to kill with Skeet loads.
For late season, I often move up to 1-1/8 oz. 7-1/2's, when the shooting can be a little longer in range. I see no reason to go to 6's but certainly they will kill .
For pheasant, typical sizes referenced are 4, 5, 6 & 7-1/2 in everything from standard Trap loads of 7-1/2 to heavy 2-3/4" or even 3" field loads.
Having had the privilege of many, many field hunts and in addition, being a field trial gunner, I've found the best all-round pheasant load to be 1-1/8 oz. of 6's in a load at about 1250 fps.
For simulated driven or "European" shoots, I've used the same loads and have even experimented with 1-1/4 and "magnum" 1-1/2 oz. loads of 5's and 6's. I don't find them necessary and prefer the standard 1-1/8 oz field loads of 6's. As to 3" loads, there is significantly more recoil and expense, and in my opinion, are absolutely not required. For close in dog work, 7-1/2's will work, but 6's seem to be much more effective overall.