Duffy -- Unless you are shooting pheasants over pointing dogs, where shot size make less of a difference, 1 ounce loads of steel #3 or #2 shot should be solid performers for general pheasant shooting. Yes, I've killed them with steel #4s and I'm sure close birds can be taken with #5 or #6 steel too but I'll take the larger shot for pheasants.
Here are some of my 20ga 3" 1 ounce load pattern numbers to give you an idea of what you might expect from #3 and #2 steel 20ga loads.
Pattern numbers from a 20-gauge Browning Citori with 28" Invector-plus barrels and Briley flush chokes (patterns average of five, 30" post-shot scribed circle, yardage taped muzzle to target, and in-shell pellet count average of five).
20 GA 3" WINCHESTER DRYLOK SUPER-X STEEL LOAD
1 oz #3 steel (145 pellets) @ 1,330 fps
30 YARDS -- SK / pattern 114 (78%)
30 YARDS -- IC / pattern 123 (85%)
40 YARDS -- IM / pattern 106 (73%)
40 YARDS -- LF / pattern 110 (76%)
20 GA 3" REMINGTON NITRO-STEEL MAGNUM LOAD
1 oz #2 steel (118 pellets) @ 1,330 fps
30 YARDS -- SK / pattern 101 (86%)
30 YARDS -- IC / pattern 105 (89%)
40 YARDS -- IM / pattern 99 (84%)
40 YARDS -- LF / pattern 102 (86%)
Good luck!