The low-temperature behavior of the resistance of polycrystalline thin films of magnetoresistive manganites has been investigated for grain sizes in the nanometric scale. We have optimized the de-sputtering deposition and annealing procedures so as to meet the grain size requirements while keeping the basic features of the magnetic and electronic transport of the bulk material. A systematic upturn of the resistance at approximately 50 K is observed in all films (12-80 nm grain diameter). These results point to the existence of a Coulomb gap affecting the conduction of electrons at least in the smallest grain sizes explored. The evolution of this effect with the magnetic field is also investigated.