I ordered my son the H & R Ultra Slug in 20 gauge. I dont have the gun yet, but want to get order him a slip on Limb Saver butt pad.

Since I dont have the gun in hand I can't measure to order the recoil pad, does anyone with one of these shotguns have this particular butt pad on their gun? If so can you tell me if it's the small or medium. I was told large is to big.

Thanks in advance.