The anion-adaptive self-assembly described here not only offers a facile approach to produce large single-molecule magnets without the need for precise manipulation of the stoichiometry of ligand-metal centers but also provides an understanding of how structural factors affect the magnetic properties. X-ray diffraction analysis reveals that a rare hexagonal metallacycle with a diameter approaching 23 Å was obtained. Magnetic investigation shows that the resulting hexagonal metallacycle behaves as a typical single-molecule magnet with double relaxation under dc field thanks to the different coordination geometry of the DyIII centers.