Choose the sandwich adapter to match your oil filter base and thread, or buy Perma-Cool's universal adapter.
Perma-Cool also has a 180°F thermostat valve so your cold oil bypasses the cooler until it needs to be cooled. You don't want your oil to be cold any longer than necessary--your oil needs to be normally warm, and 180-190 is good. You'll want the thermostat with the 3/8" NPT ports to fit the ends on 1/2" I.D. hose. (The sandwich adapter also has 3/8" NPT ports.)
Permacool talks about using their 180° thermostat for a transmission cooler, but I'd prefer Hayden's 160° thermostat for ATF.
You might wait on the hoses. Have them made at a local shop after you get the cooler mounted so you get the length just right. Be sure to use good high pressure hydraulic hose and fittings, no push-on fittings or hose clamps.
Can't help you on cooler size. What size cooler would fit in a spot with good air flow?
Ken