I'd find out what your current temps are with the stock cooler first . The new ones are pretty good sized out of the box and if they were inadequate, Ford would not have installed them in the first place??
I've got a '97 with the factory cooler and it's TINY compared to the new 'stock' ones... Yet, my trans temps rarely get over 180*, even towing my TT in the Summer.. Sure, it's gotten to 210* at times, but that's only for short periods of time on a long grade and since I'm taking my trans temp from the trans OUTPUT line (not the pan) that's the MAX temp of the fluid BEFORE it hits any cooler....
If you are still curious about the 'size' difference between the "HD" version, then just go to a Local Ford Parts desk and ask them to pull one off the shelf for you.. Measure it up and compare to yours. It's free and you'll find out what the cost of it is too..
Unless you are towing heavy on a daily basis or your temps are just really high for whatever reason (like you live near Hell or something,
) I'd hold off on 'upgrading' the cooler for now...
Mitch