64e6^(1/3) = 400, in case you were wondering. So I would take it that clinical MRI voxels measure 2 mm (400 * 5 um) usually? The article does a really bad job at explaining the number from their headline.
In the research setting, if a structural MRI is being acquired for anatomical research, they'll go below than by more than half. We acquire 2mm voxel structural MRIs since we only use it to help us find a warp from subject space to template space in fMRI analysis.
Clinical scanners often use 2mm isotropic voxels, or even 3. Clinical usage is almost a bad reference point!. Research MRI at ultra high field (7T) goes to 0.8mm isotropic and below (0.5 or 0.6 is possible).
