all my racks use 3" and all ive used for 5+ years. Its plenty of heat for them since the tubs are 16" wide. This gives them a 3x16 hot spot and thats plenty to keep warm. Their entire body does not need to come in contact with heat to warm up

11" i dont trust it on racks incase of spikes and it can cause overheating much faster due to higher wattage.