I've shot relatively few pheasants with steel starting when non toxic was required for waterfowl and for everything on federal refuges. I won't use it if lead is still an option. I've used #3s, #4s, and #6s in 12 ga. and 20ga. I've had mixed results so far at best without a dog and now with a flushing dog. I thought the #4s worked well until I cleaned the birds and found the ones I got had few hits and were mostly "Golden BB" kills. That tells me that birds I thought I missed probably carried the lighter shot away as cripples which I hate above almost all other things.
I've patterned steel of at least 10 different loads in multiple brands and with consistently inconsistent results. I've used a 20 ga with mutiple chokes from Skeet to Super Duper extended turkey tubes and have had consistently spotty patterns with clumping being a uniform fault. That's what I've experienced with my gun. I'll continue to look for something in 20 ga. that has patterns as good as the loads available in 12 ga.
Tom Roster's study of steel shot for pheasants is available on the web and has been linked here a number of times. His study found #2 as the best with #3 only slightly less effective in the 12 ga. loads he used.
A lot depends on how you are shooting pheasants. If it's long shots with a flushing dog you'll need larger size. Pointing dogs or no dog and close shots and lighter shot with higher pattern density will work fine.
Pattern your gun until you find an acceptable load at your usual ranges and don't be afraid to have a progression of loads in your gun to deal with increasing ranges. I wouldn't just take what guys say that you don't know and have no feeling for how truthful or how good a shot or hunter they may be as the way to come up with the load that works for you.
Share what you find if it's not just anecdotal.