Brian,
It depends, they make a 2 tube and 3 tube heater. I would suggest a 3 tub if you have the room. go to
http://heatercraft.com/ they have some pics and you could probably email them for the dimensions. Also consider that besides the dimensions of the box itself you have to alot for the tubes to connect to it. In the boat I just bought I have to install one and I plan to get a 3 vent one with 2 hot tubes and a rotaire vent. You can check out the configs on the website as well.
I am not sure the config. of the x-2 but I would guess that you could either put it in front of the drivers feet or in the storage behind that passengers. I would ask someone who has one to post some pics. I am sure someone will help out.