We study the formation of columnar structures of uniaxial dipoles in an external magnetic field both experimentally and theoretically. By applying an external magnetic field parallel to a thin layer of a magnetorheological fluid, we manipulate a single initial cluster of suspended colloidal particles. We find that the cluster breaks up into columns that have approximately uniform widths and intercolumnar spacings. Both the average column width and inter column spacing are observed to vary linearly with column length. The observed linear relationships between column width and spacing versus the column length are interpreted theoretically by computing the potential energy of an ensemble of closed-packed columns of spherical dipolar particles.