is 36x18 ok?

[Bogertophis]

Well i used to not worry bout supplemental heat, my my new house stays rather cold. last winter was fine but this year he went off food so now he has a hot hide (88) a cool hide(77 to 81) and a cooler hide (74ish). I dont think a extra foot would make it much harder, maybe just up the wattage of my CHE.

It depends on how much more air you're trying to keep warm- don't assume, try it first: test for at least a week without an occupant, & in winter when your place is chilly. That's the only way to know.