We report the selective area growth of N-polar GaN μ-platelets on graphene by metal–organic vapor-phase epitaxy. In a first step, GaN nanowires grown by selective molecular beam epitaxy on patterned graphene arrays on SiO2 are used as nucleation seeds. The initial radius of the graphene patches results in different optical and crystalline quality of the GaN μ-platelets due to different coalescence mechanisms. The use of large graphene patches (250 nm) with significant number of nanowire seeds promotes the growth selectivity on patterned graphene at the expense of the structural quality (presence of voids, stacking faults, dislocations, and inversion domains). On the contrary, the use of smaller patches (65 nm) allows to grow μ-platelets from a very limited seed number (<3 nanowires) with a significantly reduced number of extended defects. These observations have been directly related to optical measurements by cathodoluminescence and high-resolution transmission electronic microscopy observations performed on the same μ-platelets for the different graphene patch radii (65, 90, 250 nm). The formation of defects is discussed and supported by nucleation, intra- and intercoalescence mechanisms.