Size effects of the nanopores and nanoparticles of an aerogel on its gaseous, phonon and photon thermal conductivities are modelled and analysed, as a function of temperature and pressure. It is shown that the size effects lower the values of these three heat transport components and they strengthen as the pore and/or particle size is scaled down. we show that the gaseous thermal conductivity can be reduced to values much smaller than that of air in free space, by decreasing the pressure, increasing the temperature, and/or reducing the collision cross section of nanoparticles. The predictions of the proposed model are in good agreement with experimental data of silica nano aerogels and they could be useful for guiding the fabrication of nano aerogels with a tailored thermal performance.