I would use a 5 or 6 size shot,as for the make it is totally up to you,the faster the shot velocity the less lead you need to give em,its a bit of trial and error really but thats what makes it fun.
As for the chamber size if your gun has 2 3/4inch chambers buy 2 3/4inch cartridge as for shooting 2 1/2 inch cartridge, your gun will do this safely but the shot pattern can be disrupted by the longer chamber.DO NOT shoot cartridge longer than the stated chamber size as your gun is not proofed to do this and it could be dangerous.
chhers bud so iam just asking for a 2 3/4 in cartridge t