Transmission Cooler?

Thanks for the input, guys. I've kinda got my sights on a plate-type cooler since I've had great success with them in the past. I'm just wondering how exactly one goes about sizing one. They come in .75" and 1.5" thickness and a variety of lengths. I see they are rated in lbs. Is there a rule of thumb for sizing one to a particular combo, or is better to just get the biggest one that'll fit??? Can you go too big?