the bigger the better in my opinion. It sucks to burn up transmissions and they are a pain to take in and out on most buggies. Griffin makes some nice tranny coolers along with Setrab, and PWR
If you have the room, go to junk yard and get one out of a 2005 up superduty powerstroke they are they biggest cooler you will find and cheap . They used 2 different sizes, a 22x17 and a 22x13 . They both have 1/2 inch tubes and are a plate design . I use the 22x13 in my 4000# jeep with a 3800 stall and a manual clutch fan set up and it stays about 200 all day . I paid 75$
This is what i have been running for a few years and never had any trouble out of it . I also use one for my hydro steer also . They run about 220 bucks .