If it's in a garage and then also a cat box, I'm pretty sure you won't have to worry about too much weather-wear on it, or moisture (unless the cats go tromping around in snow and track it into the garage/box).
If that's not the issue, and you're asking about technical details like watts and stuff... I wouldn't go too high in power. Since they're already in plenty of shelter, out of the wind, I'm sure just a little heat would be plenty.
My cats only get styrofoam boxes and straw >_<