In ceramic BaTiO3 thin film prepared by chemical solution deposition, the influence of small grain size (10-150 nm) and grain boundaries on ferroelectric behavior is investigated by the studies of low-frequency dielectric response. The apparent permittivity is suggested to result from an ensemble of grains possessing different properties and volume fractions, and having a nonferroelectric boundary layer. The effective permittivity of the boundary layer is found to be close to that of an interfacial layer in epitaxial thin-film ferroelectrics, indicating possible fundamental resemblance of these layers. The maximum effective permittivity of the grain interiors is estimated to be about 1500. The observed small intrinsic grain permittivity, small Curie constant, broad dielectric peaks, and high-temperature dielectric hysteresis are discussed in terms of size-induced changes of the phase diagram.