Mesoscopic patterns of cobalt nanocrystals produced by applying a magnetic field perpendicular to the substrate during the deposition process are presented. These mesoscopic patterns markedly differ with the size distribution of the cobalt nanocrystals. Well-defined columns are produced when the size distribution of cobalt nanocrystals is low; conversely, the coalescence of columns with formations of labyrinths occurs for a large size distribution. A formation mechanism of these structures is proposed.