I started the 5'x10' SCARM thread cited above. I just finished landscaping my layout last week and it's great.
My tips on small sized FasTrack layouts are:
1) If at all possible make a 5'x10' table. Makes a HUGE difference in how much track you can fit in. I spent MANY hours tinkering with SCARM before realizing I just couldn't get a fun layout into only 4'x8'.
2) SCARM is awesome but bear in mind that it only shows you the actual rails. It does NOT show you the full width of the FasTrack roadbed. I had to shorten my spur and also extend things in a couple of spots to allow for clearances. Also don't worry if things don't line up 100% in SCARM. FasTrack has some wiggle room to it.
3) I hate plain ovals. Watching a train run in the same direction over and over again is boring for me. One thing I like about my layout (and those from Ace and Mike Donohue whose ideas I borrowed from) is that the trains can reverse direction through the use of switches and crossovers. In a small sized layout crossovers are your friend!! Crossovers also reduce the number of expensive switches you'll need to buy!
Here are some pics of my layout now that it's done. Only thing missing is my K-Line operating dairy (you can see the open spot in the front). The ground cover is FusionFiber and Woodland Scenics grass powder. The FusionFiber reduces the height of the FasTrack railbed and also eliminates the uniform flatness you get from plywood.
I am planning on expanding. I'm going to have a spur come off the front corner just in front of the tunnel exit. I'll add a 12"x 36" shelf in front of the table (where the controls now reside) which should give me room to add a culvert loader/unloader set and an operating horse corral. The kids (and me) love the operating accessories. For me they're the main reason to go with O-gauge instead of HO or N (which would have been a LOT cheaper!)