The theory of size effects in nanocrystalline ferroelectric ceramics or nanoparticle powder allowing for surface tension and depolarization field is proposed. Surface tension was included into free energy functional and extrapolation length was expressed via surface tension coefficient. The latter was shown to be dependent on temperature due to its relation to the dielectric permittivity of the nanoparticles. The depolarization field effect was calculated in the model taking into account the space charge layer on the surface. Euler-Lagrange equation for inhomogeneous polarization was solved analytically both in paraelectric and ferroelectric phases. Temperature and size dependence of nanomaterials spontaneous polarization and dielectric susceptibility were obtained. The comparison with available experimental data is performed.