It depends on your own study field. Many resting-state fMRI studies resampled their data to 3*3*3. You also can search some papers about the resampling factors. The resampled resolution is up to you. You can resample the default mask to your wanted resolution. Please see http://restfmri.net/forum/node/88 to know more about how to resample the mask or ROI files.
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