Size of heat tape/mat for a 110 qt enclosure?

In the near future I’m looking to get some B. c. constrictor, Boa imperator, and B. c. longicauda. I do plan on setting them up in 110 qt tubs in the beginning (34.62 Inches (L), 12.62 Inches (H) x 18.75 Inches (W)) and was wondering what would be the best size heat tape? Note that this will be for young animals. Would 12” be sufficient as it’s a bit over 1/3rd of the enclosure and that may provide a sufficient gradient or would 21” be better in case room temps drop a bit? Usually my invert room is 69-74 F.

Thanks!

Usually 1/3 is the standard, so I would recommend 12". If there's ever a thermostat or heat tape problem you want the snake to have an option to get away from the heat.