Cooler size is 100% dependent on use. If the car is rarely driven then it doesn't need much of a cooler. Most street/strip cars don't need much of a cooler since they typically are not driven very hard. As soon as you start doing a lot of WOT driving, or towing or hill climbing then you need a big cooler. If you don't have a tranny temp gauge then you don't know what you need. I had a temp gauge channel on my data logger in the Duster and the tranny temp was always cold when cruising around town. Only time it started to get any heat in it was freeway driving and then only after about 20 miles. But if you went WOT going up a long hill on the freeway then the temp would climb pretty fast.