Stock is 2.25". You don't want any smaller than that. I would personally go 2.5".
I removed my cat, and welded the 2.25" Super 40 in with an extension to make up for the shorter length.
It is not that loud to me. It is nice, but not too loud. It is louder than the aftermarket cat-backs I have heard. You may be a little disappointed if you get 2.25". I can barely hear mine on the highway with the hardtop on.
I had no cats on my XJ with a FM Delta 40 2.25" and it sounded mean.
To know what you like, you'll have to listen to Jeeps in real life. Videos do no justice.