If your trans lines are the screw in type like my 2005 SXT, the nut size was 18mm, open wrench.
The new radiator I ordered has the hose type fittings. The fittings look like they scew in. I haven't had time yet but I want to see if they do unscrew so I can screw in the transmission lines. If not I guess I either have to send it back or cut off the screw in fittings and use a patch hose (not what I want to do)
The radiator I ordered was csf3180 for my 2005 Neon SXT. It had two male hose end fittings for the transmission cooler. The male fittings DO NOT unscrew from the trans cooler. The nuts on them only hold the cooler in place.
My old radiator had screw in trans lins. The screw in fittings are help on with snap rings. Once I removed the snap rings the screw in fittings came off to reveal male hose type fittings. I connected the two male fittings with 3/8 transmission hose and hose clamps.