It's not necessarily about the size of the UTH but about the output wattage. You could have a small mat that's rated to put out 200w versus a huge mat that's only rated to 60w. You could buy a larger mat and just put the tank/tub on part of it, there's nothing wrong with doing that, so long as you still have a warm side and a cool side to the tank. Here are a few links of set-ups from the site for how to set up a glass tank. Don't know if they'll help any or not, but might be worth a look.

http://ball-pythons.net/forums/showt...p-w-pics-*DUW*

http://ball-pythons.net/forums/showt...Basics...*DUW*

As to getting a tub, DOOOOOOOOOO ITTTTTTTTTT, DOOOOOOOOOO ITTTTTTTTTT! HAHAHAHAHA! Just kidding, just kidding. Do what works best for you and your snake.