I think I'm going to do a bit of both because my lights are going to be mounted in the side walls so I think I will get the 11" and get two feet to mount along the back wall so they each have roughly (when you take out the amount the divider blocks) an 11" x 11" basking area. That should be big enough even for my big Loki boy who broke into 1,000 grams as of last night's weigh in! and he's not even two yet so I'll be excited to see if he's done or still got some to go.