Because it seems to be getting some good press here lately, I'll toss in the link for Google SketchUp, a free 3D design program with a very intuitive interface and some impressive capabilities.
I've been playing around with this lately, I wish more 3d programs were this easy to use, and weren't such resource hogs.
I would recommend this esp for a newbie. :thumbsup:
That Google dude is a great visual size reference.