afni.nimh.nih.gov

You can leave out the geomparent option then and supply "-xFOV 120L-R -yFOV 120P-A -zFOV 64S-I" where you would replace the numbers with half the distance in each direction (in your case 25,30.5,11.5)and putting in the appropriate orientation for Left-Right, Posterior-Anterior and Superior-Inferior, where the directions could be reversed, depending on the orientation of the dataset.