I would bet money on 1 1/2 then, extremely common racoon trap. They work fine for otter but are pretty small for a blind set, especially for someone learning to trap. Using something like a #3 allows a little more room for error and isn't inhumane on an otter, IMO. They have pretty large feet and thick legs so you aren't likely to maim them....and yes I realize the irony of not wanting to injure something before you kill it. I think a couple 220 or 330 conibears are your best bet though.
