Nanostructured ceramics of high-temperature piezoelectric 0.375BiScO3–0.625PbTiO3 were prepared by spark plasma sintering of nanocrystalline powders obtained by mechanosynthesis. The macroscopic electrical properties were characterized on dense ceramics with decreasing average grain size down to 28 nm. Results indicate that the electric field is screened by the electrically insulating grain boundaries at the nanoscale, which needs to be considered when discussing size effects in ferroelectric polycrystalline materials. Moreover, the requirement of increasingly large electric fields to achieve a given polarization with the decrease in the grain size, together with the depletion of the dielectric anomaly associated with the ferroelectric transition until its disappearance, seems to be the result of grain boundary effects.