You really only need a 'larger' cooler if your tranny temps exceed 225* for sustained periods IMO... This means you are going down a flat hwy and just 'cruising' while towing.
If your temps 'spike' during periods of different driving conditions, then that's normal and if you get to temps of 225*, but it falls back down when you get back to your cruise mode, then your existing cooler is doing it's job...
I got a trans temp gauge that is taking the temp on the trans OUTPUT line, so it's the max temp the fluid is getting BEFORE it gets to a cooler and the most it's gotten to is 210*.... And that was a 90*+ day going up a 6% grade in 2nd gear.. When I got to the top and going down the back side, the temp went back down to around 180*.
The stock coolers on the late model F150's are HUGE, compared to the earlier models, so I'd say get your current trans temps first and then decide what you need to do...
But, if you wish to upgrade for the sake of upgrading... Well, that's your choice..
MItch