honestly, i would not use anything bigger than 4 inch for belly heat. if you are wanting back heat go with 11 inch. if you are not sure of how to wire the heat tape to a thermostat, you can do the easy thing and just get yourself a rheostat, it just plugs right in and is easy for a beginner to use. make sure to check the actual temp of the heat tape, not just in the tubs. the aluminum tape not only hodes it down it also helps a tiny bit with adding aditional heat.
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1.2 ball pythons
1.2 columbian boas
1.1 thayeri kings
1.1 mexican black kings
1.1 red african house snakes
1.0 grey-banded kingsnake(loma alta locality)
0.0.1 snow corn
0.0.1 ghost corn
0.0.1 blood red corn
1.0 hypo honduran milk
0.1 albino honduran milk
1.2 texas bulls
1.0 western hognose
1.0 cave dwelling rat
1.0 bairds rat
2.0 texas banded geckos
1.2 african fat-tail gecko
1.0 savanah monitor