If it is big enough for a Norway rat, it is big enough for an ermine. We had, thankfully only 1 year, Norway rats digging under the 3 level plastic compost bin at the soon to be renovated and rented 'city' house 5 or 6 years ago. The first time I saw the critter's head coming out of the hole, I thought it was a juvenile gray squirrel, until I saw the tail. Much bigger than your pet store variety rat. Not sure why they moved on but NOT complaining that they did!
I don't know about that specific trap with regard to humaneness, but since the mice in the 'farm' house wised up to the 'free trip to the back of the barn' 5 gallon bucket no kill trap, I broke down and got a couple of snap traps. No mess and they trip so fast, I doubt there is any suffering. I got 8 or 9 over a 2 month period, no blood, no guts and CHEAP. Bear with me here, background: My daughter and the 3 indoor cats moved back to the 'farm' house 2 weeks ago (LONG rehab). It was a necessity since she doesn't have a driver's license and my wife and I took a LONG hours in the car 6 day trip to southern Wisconsin to pick up our younger daughter from college. No one to watch the chickens while we were gone! So away went the snap traps, daughter wouldn't like them, DEFINITELY wouldn't have emptied and reset them and not safe for the cats. Early morning yesterday there was a LOT of commotion for a long time. I have no idea what time it was, still very dark out and the youngest cat (my daughter's) does sometimes play at night but this was different. I'm thinking he found a 'self motivated toy'. I heard one of the cats barfing at 5:30. Yep, dead mouse and barfed up mouse guts on the kitchen floor, daughter's cat walking away. Me? If I were a mouse, I would prefer the snap trap! Daughter? Never heard a thing and I am not mentioning it. She doesn't like any animal to be killed (not even spiders which she loathes) and she especially doesn't want her cat killing the mice. She just wants them to stay outside. That would suit me fine as well but the mice don't seem to understand.
Bruce
You still had a small rat. The river rats in Nashville (they stay by the river they don't go more than a few blocks away) are decendants of Norway rats. I have seen one the size of a large cat. That's when I realized why you have cats as mousers and terriers as ratters. A cat would need to be the size of a lynx or at least a bobcat to have taken this bad boy on.