You have to be aware of what type of Tranny Cooler as well as the size. I have a Hayden flat plate tranny cooler in my 97 T-Bird. It is for 14,000 GVW. It's small, but keeps my 4R70W cold! If you are really worried about heat get a medium sized cooler, but the flat plate ones are the most efficient.
Oh, I have my cooler mounted in the space UNDER the battery, not in front of my radiator.