You can know more about your tensor image by using the command
VolumeInfo tensor.nii.gz
The output should look like
Volume Info of tensor.nii.gz size: Nx x Ny x Nz, voxel size: sx x sy x sz, origin: [0, 0, 0]
which suggests that there are Nx, Ny, and Nz slices along x (sagittal), y (coronal) and z (axial) dimensions, respectively. The voxel size corresponds to the voxel spacing in mm.