In this work, we have investigated the crystalline-size dependence of optical absorption and photolumines-cence emission of CH3NH3PbI3 films, which is necessary to identify the potencial practical applications of the gadgets based on perovskite films. This study was carried out at low temperatures to minimize the extra complexity induced by thermal effects. The purpose was clarifying the origin of the dual emission peak previously reported in literature. We have found that the grain-size is responsible of the appearance or disappearance of this dual emission on CH3NH3PbI3 at low temperatures, whereas we have inferred that the thickness of the perovskite layer is a much more important factor than the size of the grains in the location of the energy of the bandgap. Moreover, the increase in the grain size allows slowing down the phase transition. Additionally, we evidence a decrease in the effective Rydberg energy of the exciton in several samples, from 23-25 meV at 7 K to 12-13 meV at 165 K, by fitting to Elliot-Toyozawa theory. We have extracted other im-portant physical parameters of perovskites from the photoluminescence-data deconvolution, such as bandgap, exciton-phonon interaction and exciton binding energy. A new phase transition at 45.5 K was determined by the temperature dependence of full width at half maximum and integrated intensity of the photoluminescence, and it was confirmed by the radiative lifetime obtained from the time-resolved photoluminescence emission by mean of time-correlated single photon counting at different temperatures, excitation fluencies and emission energies.