Well, most of the loonies around here are person sized, but you probably mean something else. I've seen trapdoor snails the size of a walnut before. Maybe 1" diameter (2.54cm).
I've got yoyos, and they really dig on snails. However, they need to get snails that aren't too small to geth their mouths on, nor too large to pull from their shells. Generally, when I throw a couple dozen pnd snails in there, I find a couple escape and go on to breed and grow. No infestation - the loaches get most of the eggs before they hatch and most of the snais when they're at the right size, but some of them survive.