Largest shoulder fired gun: 40mm grenade launcher.

Largest "normal" rifle: A muzzle loader when I was 12 that was a replica of an old Norwegian gun. Don't remember the calibur, but somewhere around .50 cal.
This is the only rifle that really brused my shoulder. The owner loaded it pretty stiffly most likey, as I was blue almost from my elbow to my neck... Being just a stupid kid, I was quite impressed, and rather proud, as I even hit the rock I was aiming at...

Erik D.

Seems I forgot about the 84mm CG that Tunofun posted and thus reminded me of. So that was obviously bigger than the 40mm. But then I wouldn't perhaps call the CG a "rifle shaped shoulder fired weapon" which is more in line with the poll! Even though you do use your shoulder...