Radius size for sphere seed ROI for GPPI

Hi AFNI gurus,

I was just wondering if there was a method of choosing the radius of an ROI sphere? I can’t seem to find a methods paper that addresses this? Would one typically use the lowest EPI spatial resolution or FWHM size? Or some factor of the former (e.g., double the lowest or double the FWHM)?

Thanks!,
Ekarin