I actually have two tracks around the perimeter of my son's bedroom (13'x13') about a foot down from the ceiling so there's no scenery but I can run some decently long trains. I hope to get to a nice layout in the basement someday.
When I was little my bedroom had a 6" thick ledge almost all the way around. I always wanted to put a good train set on it. My friend was into trains for a while and some of his set ups were pretty sweet. I remember the mall use to have this massive one that you could walk up to and put a quarter in and choose a set that you want to watch. One time my class went on a field trip there and had every train running. I was amazed for the 5 min that they all ran. It was like watching a town from the sky.