http://www.bigappleherp.com/Zoo-Med-...=2&category=14
If the ambient in the room is fairly high I would get the medium 8"x12", if the room is fairly cool get the large 8"x18". The large is a bit oversized for the tub - you want to try for covering 1/3 of the floor space - but there will still be plenty of space available for a cool side even with the large pad. Or you can put the water bowl over part of it, which may keep it humid enough inside that you never have to spray.