It depends on how you are going to use it.
1. Are you using it as belly heat?
2. Are you using it as back heat?
I suggest 3 or 4" for the belly or 11" for the back.
I have found the belly is more direct heating but over time the heat tape wears down due to the constant rubbing across it (which can cause the tape to short.)
I like the back heat best. The only draw back is it not as direct in heating but the size (11" ) makes up for it and it does not wear from being rubbed on.
Good luck, Mike
