All of the turkey hunters I know use a 12 (including me). I do know some goose hunters who use 10's. Ammo can be a bit pricey on 10's and you may not find it at your local store. But then again ammo prices on 3 1/2 inch heavey shot aint cheap either.
I have had good luck in the late afternoon both spring and summer. I learned atrick recently. After the toms mate (late in the season), it is harder to cal them in. A buddy of mine uses an owl call. He says when he calls with the powl call, any toms nearby usually gobble. Don't know why, but iy=t is a good way to locate them
Good luck
okcmco
PS. I use a cheap winchester 1400 semi-auto with 2 3/4 inch shell. I would like to use a magnum or super mag but have not been able to afford one yet.