Nanofluids (or dispersions of solid nanoparticles in a liquid) have attracted considerable attention for high performance heat transfer, because large enhancements in the effective thermal conductivity have been reported when small amounts of solid nanoparticles are added to common liquids such as water and ethylene glycol. However, there is considerable disagreement in the literature with respect to the temperature and particle size dependence of their thermal conductivity. In this work, we show that the thermal conductivity - temperature behavior closely follows the behavior of the base fluid. Many published models cannot reproduce this behavior, mostly because limited data sets were used in their development. In addition, we also shows that the thermal conductivity enhancement for nanofluids increseas with increasing particle size and reaches a limiting value. Our results support the hypothesis that quantum effects dominate heat conduction in nanoparticles, which creates a size dependence for the thermal conductivity of the solid in the nanoscale regime. This dependence is most likely due to phonon scattering at the solid-liquid interface. The limiting value of the enhancement is greater than that predicted by the Maxwell equation, but very close to the volume fraction weighted geometric mean of the thermal conductivities of the two phases.