Aluminum or Stainless. Your normal muffler shop stuff will be steel and wont last. Aluminum is nice but you would have a much larger outer diameter than stainless for the same inner diameter. Stainless would be best for utilizing the not-a-lot of space you have under the tray, but aluminum would be ok too.
As for the size... Measure the largest inner diameter you plan to put in your exhaust system. I believe the stock X2 exhaust system is 1-3/4" or 1-7/8". You exhaust tube should be at a minimum the same size (on the inside of the pipe) as your waterbox. I upgraded to a B-pipe and everything is 2", so I also went to 2" outlet so as to not restrict the pipe. If you think you might upgrade your exhaust someday to a 2" ID system, put in a 2" ID rear exhaust.