Grain　size　effects　in　ferroelectric　ceramics　have　long　been　exploited　to　tailor　their　functional　properties.　However,　the　underlying　mechanism　for　the　grain　size　effects　is　not　yet　fully　understood.　Here,　we　study　the　grain　size　dependence　of　domain　wall　activities　in　a　lead-free　piezoceramics　system,　(Ba,Ca)(Zr,Ti)O3,　with　grain　sizes　in　the　range　of　4　-　22　mu　m.　The　dielectric　permittivity　is　highest　at　intermediate　grain　sizes　(-12　mu　m)　where　there　are　moderate　lattice　distortion　and　the　most　active　domain　wall　motion　under　low　voltages.　Despite　the　larger　fraction　of　switched　domains　in　the　material　with　larger　distortion　(-22　mu　m),　as　revealed　by　in　situ　electric　-field　synchrotron　X-ray　diffraction,　time　-resolved　characterizations　demonstrate　an　easier　and　faster　domain　wall　dynamics　in　the　sample　with　moderate　lattice　distortion.　Our　analysis　and　phase　-field　simulations　show　that　the　grain　size　dependence　of　the　domain　wall　dynamics　of　polycrystalline　ferroelectrics　is　dictated　by　the　interaction　between　an　external　electric　field　and　the　grain　size　-related　change　in　intergranular　stress,　and　this　interaction　is　most　effective　in　stimulating　the　movement　of　non-180　degrees　domain　wall　at　intermediate　grain　sizes　during　the　initial　phase　of　polarization　reversal.　Our　results　provide　a　new　fundamental　understanding　to　guide　the　future　design　of　materials　for　improving　functionalities.