I use 11" on my 32 quart tubs. My rack, which is a less-than-stellar homemade one, is open sided and backed and as such loses quite a bit of heat. Something else to consider is how hot you would have to run the 3" or 4" tape if you elect to use that. The nice thing with the 11" is that it's a higher wattage rating per foot and doesn't have to run at max capacity to achieve the proper temps. Just my two cents...

Good luck, whatever you decide!