I have a question about racks and flexwatt

[twh]

I am building a 9 tub rack using vision v35 tubs.
http://www.visionproducts.us/htm7/tubs.html

I will be using flexwatt (across their width) to heat them.
what size would be best to use?
3", 4" or 11" ?
thank you very much
Bill

closed or open air rack ?

room temps ?