I'd find the volume of the stock setup and come close to matching it.
If he's deleted and a manual, then he won't have the egr cooler, or the trans heat exchanger that account for a lot of heat to the system.
Also, I'm assuming if he's towing ~20k and going through mountains, he's got a charger setup to keep him cool, and not a drag racing setup.
And there's some efficiency to be had by using an all aluminum radiator like a Griffin or Mishimoto (I personally am really impressed with Mishimoto. Quality and warranty).
Oh, and surface area is more important that thickness, but it's a small factor compared to overall volume.