The zero-shear-rate viscosity eta(0) of polymer-nanocomposites (PNCs) derived from entangled poly(dimethylsiloxane)s and rigid polysilicate nanoparticles were investigated as a function of molecular size and concentration. Narrow molecular weight fractions of polymer and nanoparticle were obtained by supercritical fluid extraction. Molecular weight properties were analyzed by size exclusion chromatography and the nanoparticle radius of gyration R-g was characterized by small-angle neutron scattering. All seven polysilicate fractions were smaller (0.75 <= R-g, nm <= 2.1) than the five polymers (3.0 <= R-g nm <= 12). Relative to the polymer eta(0) the PNC eta(0) exhibited either plasticization (viscosity reduction) or reinforcement (viscosity increase). Only reinforcement was observed in PNCs based on the polymer below M-c-the critical molecular weight for chain entanglement effects to start influencing eta(0)-which included an increase in the PNC eta(0) by over 3 orders of magnitude using 0.30 volume fraction of the largest nanoparticle. For polymers above M-c, the crossover from plasticization to reinforcement behavior could be described by a critical molecular-size ratio based on the unperturbed R-g of the nanoparticle and of the polymer. Viscosity reductions of up to 52% were achieved, and were more significant at the higher of the two nanoparticle concentrations studied. The critical nanoparticle-to-polymer R-g ratio at 298 K was 0.18 +/- 0.006 and 0.13 +/- 0.003 for a nanoparticle volume fraction of 0.17 and 0.30, respectively. A generalized form for the concentration dependent crossover ratio is proposed to account for perturbations in the molecular size of the PNC components that can be the basis for future studies. The effects of particle size polydispersity and temperature are discussed.