fslroi uses voxel, but would not be the right tool for your purpose 
anyway, since it changes the image size, cropping it to the defined region.

If you want to create a cubic mask you can use the -roi option of 
fslmaths. This option uses voxel coordinates, so you have to adjust the 
values according to your voxel dimension.

If you want to create a more sphere-like ROI you can do follow this 
description in a recent post:;6ce4bff3.1301

I hope this helps,

On 04/16/2013 12:52 AM, Nucleos P wrote:
> Hello Experts -
> When using fslroi it is unclear if the x size, y size, and z size
> refer to mm or # of voxels around the coordinates?
> For instance, if I wanted to create a mask at ROI 125, 97, 107 MNI 152
> voxel coordinates and then extend to points outside (e.g. 8mm) how
> would I set this up?
> Thank you,
> Cat