No, depending on the model, they are not spherical and the local tissue properties are used to inform them.
For your purpose maybe the Mädler & Coenen model is helpful which gives you a diameter depending on Voltage and Impedance.
Mädler, B., & Coenen, V. A. (2012). Explaining Clinical Effects of Deep Brain Stimulation through Simplified Target-Specific Modeling of the Volume of Activated Tissue. AJNR American Journal of Neuroradiology. http://doi.org/10.3174/ajnr.A2906
Function that calculates the R is maedler12_eq3 inside ea_genvat_maedler
-> In case you want to get more familiar, why not have a look at this walkthrough video: http://www.lead-dbs.org/helpsupport/knowledge-base/walkthrough-videos
It’s not too complicated :)