The relationship between grain size and domain size distributions has been studied by piezoelectric force microscopy in ferroelectric films with average grain size of 150 nm. As the ratio of domain size to grain size increases, the domain size deviation decreases in a 1/xn-type function, where n is 1.105. Extrapolation of the model shows that in order to obtain 10% domain size deviation in 1 Tbit/in.2 media, a grain size smaller than 14 nm is required. The obtained results imply that either nanograin or single crystalline/epitaxial films provide reliable domain distributions for probe storage applications.