For Giant CA geese, T is deadly. For Greaters, BBB works well. For lessors BB works well. All this is for steel, and make sure you get the High Velocity loads (over 1450fps). Some guys like to use 1 size smaller shot than what I like to use.
Over the decoys- Improved cyl, skirting the decoys Mod and longer shoots Imp Mod should work for you.
Hevi shot is deadly on geese - even giant geese, but is expensive. You better be a good shot or your pocket book will be a hurting.
As for size of shell, I use 3.5", but use the biggest shells your gun can safely handle so you have a bigger HV payload for a better pattern density.
Well this should be a start for you, hopefully a few others will chime in on thier experience too.