Transmission Cooler?

As been said, you can't go too big - especially not in the Central Valley and with a smaller than stock converter. Go as big as you can make fit. Stacked plate are good, stacked plate with fins (Setrab brand) are better. I would plumb it so that aux cooler first, then radiator cooler. That small converter is gonna make some heat, dump as much of it as you can before going into the radiator.

There is ATF cooler specific hose made, use it if you don't use braided SST or FC-332 type hose.