This is a medium sized trap, about 16" long and 6" high / wide. I've used it for various rodents including a squirrel and a small possum (see 2nd photo above!)
It works pretty well, just put in your choice of bait (peanut butter seems to work well) and keep an eye on it. You can safely transport the critter to somewhere else (perhaps a neighbor you don't like?) and let him go.
Make sure it is big enough for the suspected pest ... sometimes people rent it and then decide it's probably too small.
I also have a larger trap, for raccoons, dogs, and other large animals, here: http://www.loanables.com/items/show/975
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