If you live in a hot climate or routinely travel at high speeds thru high temps, or carry heavy loads, bigger is better. The aftermarket cooler I have on my 05 SEL Premium Duratec is rated for 9000 lb GVW (i.e.,total overkill) and is around 15" wide by 10" tall. 2 years ago. driving thru South Dakota on I80 cruising at 85 mph (speed limit of 80) with the ambient temps of 105 deg F and AC running to keep the car interior at 72 F, tranny fluid was hovering around 200 deg F as shown by Forscan on my ipad. I have a slightly smaller tranny cooler on my 86 Tbird 5.0 L V8 and have a digital trans temp gauge. Never seen trans temps exceed 200 deg F in the 30 years since I installed the temp gauge and tranny cooler even in very hot temps driving at high speeds.