How to find the diameter / surface or the middel surface?

Hello,

I have a problem with my grasshopper model.

It is a bridge, with a road in it.

One of the variable items is how many cirkels around the bridge. I can select the surface of the middle cirkel, but not the cirkel himself. So I need to know how to calculate the diameter or the surface of that cirkel.

I need that to link that coordinate to the shape of the road. If I change my bridge, I want that the road is changing automaticly correctly with the bridge.

I hope you'll understand this question and you can answer it!

Thanks.