It's difficult to "over-cool" a working transmission. I use the biggest one that will fit the space I have available. On some really hard-working rigs like a vehicle that will tow a big camping trailer often, I have used an entire air conditioner condenser as a trans cooler, but that's probably a little overkill.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!