Ferroelectric nanostructures have recently attracted much attention due to the continuous demand of miniaturizing devices and discovering novel phenomena. Therefore, in depth study of the ferroelectric, and dielectric properties at various particle sizes in nanoscale ferroelectrics is of technological importance. Unfortunately, research aimed at discovering and understanding the size effect on the electric properties of ferroelectric nanostructures are scarce. Here, we investigate the spontaneous polarization and dielectric constant at various sizes of nanocrystalline potassium niobates (KNbO₃) by using powder X-ray diffraction (XRD) Rietveld refinement, Raman spectroscopy, and theoretical calculation. It is surprising to find that the spontaneous polarization P_s reaches the highest value of 40.3 μC cm⁻² at 100 nm, and decreases with increasing grain size from 100 nm to 500 nm. The dependence of the dielectric constant on the grain size shows similar variation as that of spontaneous polarization. It is deduced that the large lattice distortion induced by lowering the grain size facilitates the increase of P_s, which in turn contributes mainly to the enhancement of the dielectric constant.