[en] The microstructure, crystal structure, and magnetotransport properties of microsized and nanosized Badoped manganites have been investigated. A 'two-step' reduction-reoxidation procedure has been used to obtain nanosized ceramic manganite Nd_0.70Ba_0.30MnO₃ (II). The parent microsized manganite Nd_0.70Ba_0.30MnO₃ (I) was prepared by usual ceramic technology in air. Then the sample was annealed in vacuum. The grain size of the reduced sample, determined by scanning electron microscopy, decreased from ∼5 μm down to ∼100 nm. To obtain the oxygen stoichiometry nanosized sample, the Nd_0.70Ba_0.30MnO_2.60 was again annealed in air. It is established that the (I) sample is a pseudocubic perovskite, whereas (II) is tetrahedral as a consequence of Nd³⁺ and Ba²⁺ ions as well as the ordering of oxygen vacancies. The (I) sample is a ferromagnet with T_C ∼ 140 K. It has metal-insulator transition at T_MI ∼ 135 K and a peak of magnetoresistance ∼50% in a field of 9 kOe. For the (II) sample, the critical points of phase transitions move to higher temperatures, T_C ∼ 320 K and T_MI ∼ 310 K. The magnetoresistance of the (II) sample at room temperature (T ∼ 293 K) is about 7% in a field of 9 kOe. The magnetotransport properties are interpreted in the framework of the nanosized effect