This work comprises an experimental investigation of thermal intensification in polymeric nanocomposites. Polyester and epoxy resins were employed as continuous phases and alumina spherical nanoparticles with different diameters (30–40, 27–43, 150, and 200 nm) were used as fillers, in volumetric concentrations up to 10%. The thermal conductivities of the fabricated samples were measured using a guarded heat flow meter and the influence of filler size was investigated. Density measurements were performed for verifying the volume concentrations of particles in batches containing distinct sorts of nanoparticles. In addition, scanning electron micrographs of all samples were obtained. Results show that larger particles are responsible for larger agglomerates and greater thermal conductivity augmentation of the polymeric matrices, indicating that interfacial properties play a significant role in the effective thermal conductivity of nanocomposites.