In the past decade, ferroelectric materials have been intensively explored as promising photocatalysts. An intriguing ability of ferroelectrics is to directly sperate the photogenerated electrons and holes, which is believed to arise from a spontaneous polarization. Understanding how polarization affects the photocatalytic performance is vital to design high-efficiency photocatalysts. In this work, we report a size effect of ferroelectric polarization on regulating the photocatalytic overall water splitting of SrTiO₃/PbTiO₃ nanoplate heterostructures for the first time. This was realized hydrothermally by controlling the thickness and thus spontaneous polarization strength of single-crystal and single-domain PbTiO₃ nanoplates, which served as the substrate for selective heteroepitaxial growth of SrTiO₃. An enhancement of 22 times in the photocatalytic overall water splitting performance of the heterostructures has been achieved when the average thickness of the nanoplate increases from 30 to 107 nm. A combined experimental investigation revealed that the incompletely compensated depolarization filed is the dominated driving force for the photogenerated carrier separation within heterostructures, and its increase with the thickness of the nanoplates accounts for the enhancement of photocatalytic activity. Moreover, the concentration of oxygen vacancies for negative polarization compensation has been found to grow as the thickness of the nanoplates increases, which promotes oxygen evolution reaction and reduces the stoichiometric ratio of H₂/O₂. These findings may provide the opportunity to design and develop high-efficiency ferroelectric photocatalysts.