Evaluation of rhino made geometry

I´m working on a school project that has to be underground. The site is a park that has some paved areas so I´m developing a definition which evaluates the area occupied by the roots of the trees (defined by its height, trunk diameter, etc...) so a volume can be generated which do not interfere with them. I have already mapped the trees directly on rhino but I don´t know how to evaluate its diameter from grasshopper for instance. Any suggestions?

Thanks