What size tranny cooler?

[Kukur]

Want to service my tranny (flush and filter) and would like to add a tranny cooler at the same time.

Been seeing different sizes (by GVW)...

Trans Cooler
16,000 GVW (11'' x 5.75'' x .75'') $42.99
19,000 GVW (11'' x 7.5'' x .75''). $45.99
24,000 GVW (11'' x 6'' x 1.5''). $49.99

Is there ANY significant reason why I would need to go bigger than the 16k?

This is being installed on a 78' Wagon (Around 6k GVW I believe? :confused: )

Thanks

[Stuka]

bigger the better!