Say your running a 3/8 inner diameter trans line. If you buy a cooler that has bigger inner diameter tubes/holes, it will make the fluid velocity slow down and cool better. Especially when driving at slower speeds. Think of it like a lake with a dam on it. If you put hot water into the lake, it will take a super long time to make the lake water hot.
On another note, you dont want trans fluid to cold or hot. I like it between 165-195 degrees, in the trans pan temp. Hope I helped ya