which would create a block 10x10x20. As far as positioning and putting things together, I think you could take a cue from SketchUp and do intelligent things with endpoints, faces, etc. Hmmm... maybe I just want to download POVRay again.
Update: Such a thing kind of exists - Visual for Python lets you do almost what I say in the post above:
floor = box (pos=(0,0,0), length=4, height=0.5, width=4, color=color.blue)
ball = sphere (pos=(0,4,0), radius=1, color=color.red)
ball.velocity = vector(0,-1,0)