The first is the number of voxels, the second is the size of a single voxel.

From: <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of yusif Al-kheder <yusif93@live.com>
Reply-To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Date: Wednesday, March 22, 2017 at 9:31 AM
To: "freesurfer@nmr.mgh.harvard.edu" <freesurfer@nmr.mgh.harvard.edu>
Subject: [Freesurfer] Dimensions question

For a skull-stripped anatomical nifti file, What’s the difference between '91 x 109 x 91’ and 2mm x 2mm x 2mm?
image1.png
Youssif