i hunt wild pheasants and use steel for them. I typically use a mod choke with 3" 6's, which are usually left over from teal season. I think 2's, 3's and even 4's are a bit over kill for a pheasant. 5's are perfect, but i have a hard time finding them. Here is something you need to consider, part of the reason such large shot is necessary to bring down large waterfowl is because they have so much down which absorbs a lot of the shot's energy, couple that with massive layer of body fat and you need a larger shot to penetrate. pheasant don't have such a heavy and dense layer of feathers, nor do they have the body fat of a duck. in over 4 years of hunting pheasants with steel, i've never lost one! And i do enjoy the higher speeds steel comes in.
one caveat to this, if you think the birds are going to be wary and flushing at 25 yds or more, then consider using a larger shot. the birds i hunt tend to hold well or flush so far out of range anything less than a howitzer wouldnt bring them down anyway.
"That's a big bird." "Really i thought he'd be more yellow!"