Question- How do I know what size trap?
Answer- Find the tunnel, which means digging down past the mound and get to the tunnel that is parallel to the surface. That is the animals run or main tunnel. That is where you will be putting the trap. The jaws of the trap when set, should fit snug into the tunnel. It is better to get a slightly larger trap than your tunnel size than too small. Make sure the jaws are not resting at a bend in the tunnel, because then the animal can get around the trap as well.