I've got a similar setup. THG heat tape and a Herpstat 1. To determine what width and length you will need you have to consider the size of the animal, the size of the tubs, and what temperature is the room the rack will be in. I have a 6 tall 41qt rack, and each shelf has an individual piece of 12" wide and 15" long piece of heat tape. I chose the biggest because the rack is going into a relatively cool house (65-70 degrees). This way there is such a large surface area producing heat it will warm the air and provide an adequate cool side.