I saw a post earlier about a shelve layout in the author's son's room, so I though I would post some pictures of mine. Since my layout is holiday only, my dad purchased and built this for my brother and I in 2011 (I think). It's a product of Ceiling Train Kit (http://ceilingtrainkit.com/), which makes Western Style supports for your track to run on.
The loop is a square, with about 31 inches on each side. All track is Lionel Fastrack, and all curves are O-72. The track is powered from an MTH Z-750 conventional transformer. Of course, space for the trains was an issue at first, so we installed 5 shelves on the ceiling to hold some of my trains that go straight to the shelves rather than back in their boxes when my train layout comes down. Here are some pictures:
