UTH Size? Between sizes

[The Night King]

Hi gang-

I'm reading the sticky for setting up my tank and I'll be using the Exo Terra 24 inch long terrarium. I know the recommendations for UTH say to get something about a third width of the tank. I'm looking at the ultratherm UTH. I think they have 6x11" and 11x11" neither is really 1/3. One is 1/4 and the other is just under a half. Room temp where the BP will be is about 72 degrees. Should I get one size over the other?

We we will be using a thermostat as recommended in the setup.

First BP for me and my 6yr old daughter