Hi all,
Some work my colleagues and I did investigating how many subjects should be included in a study has been recently published in Human Brain Mapping ( http://www.ncbi.nlm.nih.gov/pubmed/22807270). The paper should be freely available on pubmed central fairly soon if you don't have access to Human Brain Mapping early view. Or I can send you a copy.
I've provided some instructions on how to carry out a similar analysis using your own Freesurfer-processed control data here: http://www.brain.org.au/software/cortex/power
The R package provided at the above link contains a function "load.mgh.R" which can be used to load mgh surface files into R & do whatever sophisticated statistical analysis takes your fancy.
Let me know if you've got any questions.
Best wishes Heath