Kirkman,
For what it's worth, I'm building a 5x8 on a 4x8 Harbor Freight trailer. I used 2x2s with 1/2 inch ply as the floor. I wanted to keep the weight down but still have a structurally sound trailer. I used a few more crossmembers than in Mike's floor just because they were 2x2s. I've been pretty impressed so far because I've experienced no sag or shifting. The wheel wells extend about 2 inches into the cabin since I mounted the axle under the springs instead of on top.
The lesson I learned was to not overbuild (my natural tendency)!

Best advice someone gave me here is to put together a test sample of what you are thinking and try it out. I was surprised at what worked. If the wheelwell issue is the concern, Grant has a great idea with the floor storage thing. My walls are 4' high and I have plenty of cabin space. Of course it depends on what you're wanting to do in there too!
Build it the way YOU want and have fun doing it!
