On WILD pheasants, flushing at longer range on average, many years of experience taught us that a 12 gauge with 28" barrel, 3/8oz #5 shot was the ideal setup. #4 shot was too thin of a pattern at longer ranges, and #6 shot left too much shot in the breast meat.
If you hunt where birds hold for a point rather than run until forced into the air, lighter loads or lighter shot may be acceptable. We once went to a "bird ranch" where you practically had to kick the birds off the ground, and our standard loads described above were way overkill. Some birds were inedible.