UTH size?

so i was on reptilebasics.com and im looking to buy a UTH, but i dont know what size to get. i will probably have a 20L tank, so what would be the best size to cover 1/3 the tank?

A 20L is @30" long, so the 11x11" Ultratherm pad would be fine for it.

yea, anywhere from 1/3-1/2 of the tank should work great....b/c you have to figure you cool side is gonna have a water bowl/dish in its place to take up the real estate :)