3" is just fine the x-pipe is an excellent idea too,on my duster I have 3" h-pipe with cutouts into 3" mufflers with 2.5 tail pipes W/stainless factory tips..
most say thats the way to go to keep the exhaust speed up.. by the time it gets to the tail pipes its cool enought that a 2.5" tail pipe is better then a 3" tail pipe.