Metal-organic frameworks (MOF) have emerged as promising tailor-design materials for developing next-generation solid-state devices with applications in linear and non-linear coherent optics. However, the implementation of functional devices is challenged by the notoriously difficult process of growing large MOF single-crystals of high optical quality. By controlling the solvo-thermal synthesis conditions, we succeed in producing large individual single crystals of the non-centrosymmetric MOF Zn(3-ptz)2 (MIRO-101) with deformed octahedron habit, and unprecedented surface areas of up to 37 mm2. We measure the polarized UV-visible absorption spectrum of individual Zn(3- ptz)2 single crystals across different lateral incidence planes. Millimeter size single crystals have band gap Eg = 3.32 eV, and exhibit anisotropic absorption in the band edge region near 350 nm, whereas polycrystalline samples are fully transparent in the same frequency range. Using solid-state density functional theory (DFT), the observed size dependence of the optical anisotropy is correlated with the preferred orientation adopted by freely rotating pyridyl groups under conditions of slow crystal self-assembly. Our work thus paves the way for the development of optical polarization switches based on metal-organic frameworks.