The size effect of the polydopamine (PDA)-coated BaTiO₃ (BTO) (BTO@PDA) nanoparticles (NPs) on the interfacial compatibility between BTO NPs and the polymer matrix and the resultant piezoelectric performance of the composite films remain elusive. In this study, BTO and BTO@PDA NPs of various sizes were incorporated into a P(VDF-TrFE) matrix to prepare two series of P(VDF-TrFE)/BTO and P(VDF-TrFE)/BTO@PDA composites. Subsequently, the effects of the NP size on the dielectric, ferroelectric, and piezoelectric properties of the composite films were comprehensively studied. As the size of the BTO@PDA NPs increased, residual hole defects were clearly observed in the cross section of the composite film. The deteriorated interfacial compatibility due to the large size of the BTO@PDA NPs was also confirmed by the increased dielectric permittivity of the composite film, which was induced by the intensified interfacial polarisation. The P(VDF-TrFE)/BTO@PDA composite with NPs of the smallest size (100 nm) exhibited superior piezoelectric performance owing to the excellent interfacial compatibility between the fillers and the matrix. The piezoelectric performance was significantly enhanced by the reduced leakage current during electrical poling and reduced trap charges. Finally, the pulse waveform originating from the radial artery was precisely measured using the optimised P(VDF-TrFE)/BTO@PDA composite film.