Spatial clustering with spdep::skater

Dear Experts, 

My apologies in advance as this is more a statistical question, rather than an R programming question, but hope you can point me in the right direction. 

I?m working with spdep::skater to fit clusters to spatial data subject to contiguity constraints. This function fits clusters by edge removal from Minimum Spanning Trees. 

In this context, I?d appreciate any pointers on how to tune the number of clusters. What is a sensible criteria to use?

Thanks,
Axel.