[en] The aim of this research is to elucidate the size effect of magnetic nanoparticles on the resultant magneto-dielectric properties of polymer nanocomposites at radio frequencies. The block copolymer of [styrene-b-ethylene/butylene-b-styrene] (SEBS) was utilized as a matrix for the templating of magnetic nanoparticles. Surfactant-modified iron oxide (Fe₃O₄) nanoparticles of various sizes were successfully synthesized by a seed-mediated growth method. The surfactant prevented Fe₃O₄ aggregation and provided compatibility with the polymer matrix. The nucleation and growth of Fe₃O₄ nanoparticles was controlled by changing the concentration ratio of surfactant to iron-precursor. The free iron ions present during synthesis are the major factor contributing to the growth of larger particles. The Fe₃O₄ nanoparticle critical size for superparamagnetic to ferrimagnetic transition was determined to be near 30 nm at room temperature. The dielectric permittivity (ε_r) of the polymer composite increased with increasing amount of Fe₃O₄ doping, and was not influenced by nanoparticle size. However, the magnetic permeability (μ_r) of the composites was significantly influenced by the size of Fe₃O₄ nanoparticles templated within the block copolymer matrix due to thermal energy fluctuations from the nanoparticle surroundings