The thermal conductivity of a glass can be obtained by first-principles molecular dynamics provided we exploit a methodology that has been termed the approach-to-equilibrium molecular dynamics (AEMD) [1] [2, Chap. 8]. In the present work, we investigate the occurrence of size effects by comparing the thermal conductivity of two g-GeTe 4 atomic models of different sizes. This issue is far from being trivial since, in principle, size effects are not expected to occur in disordered systems beyond a few interatomic distances. For this reason, it is important to search unambiguous pieces of evidence substantiating this point. The first system of length L = 18 Å contains 185 atoms. By duplicating it along one direction we form the second system, that contains 370 atoms and features a double length L = 36 Å and an identical cross section. The thermal conductivity increases by a factor 3 from L = 18 to 36 Å, thereby approaching the experimental value. Our investigation exemplifies the crucial role of the system size to take full advantage of the AEMD methodology and bring the calculated values in better agreement with experiments.