Heat pad size vs tub size

Hello,
I am trying to buy Ultratherm Heat Pads from reptile basics. I will be using 50 qt tub, 33 L x 19 W x 7 H .
I have no idea which size heat pad to buy for this tub. Was thinking of either 11 x 17 or 11 x 11.
Do bigger pads give better heat to the cage?
So left is how it should look like with 11x17 and right is with 11 x11. Sorry for bad drawing haha.
https://gyazo.com/2313d380d5e7c7af0ae2cd8f682593fb.png
https://gyazo.com/2313d380d5e7c7af0ae2cd8f682593fb

11x17, so it can go all the way across the 19" side of the tub. This gives you a ⅓ hot side. 11" wide is more than I use, but if you keep it all the way on one side, you will still have a heat gradient, and cool side.

A properly regulated UTH source will do very little for ambient temperatures.
The larger will give more surface area for your animal to spread out on.