Just introducing myself to the board. Hope to pull the trigger on the X5 very soon. My room is 20x15x8. That should be okay right or save up a little longer for the X3s?
My listening space is about that same size and my new X5's sound great. In fact you are in better shape as the 15 foot width will allow you more room between the side walls and your speakers than I have.
You will want to have the front wall be the 15 foot dimension otherwise once you pull the speakers out 3 feet you will be forced to have your listening position too close to the rear wall.
PS. Be sure to read all of the posts here about break-in. It is real