UTH size for 15 qt tub?

[Exotic Ectotherms]

I use a 6"x11" Ultratherm heat pad from Reptile Basics on my 15qt tub. http://www.reptilebasics.com/ultrath...uth-6-x11.html

I like them because they are not sticky...you tape them on with aluminum tape and you can reuse them. I'll try to post a pic later.