Right now in my CB-70 racks I have two strips of 4", but I am not happy with the way they maintain the heat.
In my 32 qt rack I used 1' of 11" with a couple inches hanging out past the back to put the temp probe on. This rack has maintained heat perfectly and you can ask my wife, I am pretty anal about making sure all temps and humidity are perfect.
I also built a 5 qt shoe box rack and for it I used the 4". It maintains well also now, but I had to put a lot of vent holes in the front to get the cool side temp and humidity down to acceptable levels. I could have used the 3" for this project, but I plan on building a 15 qt rack eventually and would like to standardize the sizes I use and I thing the 4" will be perfect for it. So I will probably use 11" on all 32 qt and CB-70 racks in the future and 4" on all 15 qt and 5 qt racks.
I have a post on my hobby center where you can see all three racks I built.