I'd think it would depend on how you're planning to run it. If you're doing belly heat I would go with 11" for sure. I have 4" on my RBI racks and that makes for a nice toasty warm spot plus gets the cool side up to a decent temp. It's mounted 3" from the back of tub and works extremely well. If you were doing back heat 17" might work but you'd run the risk of maxing out your thermostat depending on what you're using.