FeO edge-truncated cubes, corner-truncated rhombic dodecahedra, and rhombic dodecahedra with three different sizes were solvothermally synthesized. Besides in-house X-ray diffraction (XRD) compositional confirmation, synchrotron XRD patterns reveal the presence of bulk and surface layer lattices for all the samples, with the 179 nm rhombic dodecahedra having a thick surface layer. Truncated cubes and 179 nm rhombic dodecahedra show a cell constant difference of 0.18%. High-resolution transmission electron microscopy (HR-TEM) analysis manifests significant lattice spot deviations in the surface layer region. These crystals present size- and surface-dependent band gaps. While all the measured samples have large saturation magnetization numbers, the 179 nm rhombic dodecahedra have the largest saturation and remanent magnetization and coercivity values at both 5 and 300 K, followed by the 414 nm rhombic dodecahedra and truncated cubes. Both the optical and magnetic behaviors can be understood to arise from their interior lattice variations, which should be applicable to other magnetic ionic crystals.