Perfectly circlular walls/roads in CD3? Establishing scale in CD3?

I was wondering if anyone has some tips on how I can make a perfectly circular wall or road in CD3. So far I have tried making walls 'smooth' and tracing around a circle, but that doesn't seem accurate enough. I have also tried using the 'circle' button, but I don't know how to make it produce a circle wall.

Another more general question I had was about scale. How can I establish scale in my maps when working with CD3? The closest thing I can do is to lay down a 5' road and sort of base my map off that. When really what I want is much more accurate measurements.

Thanks in advance for any help.