Thermal conductivity of amorphous and glassy materials has received significantly less attention than in crystals. Furthermore, the mechanisms underlying thermal conduction in disordered materials are considerably less well understood. With regard to the field of nanomaterials, a decline in thermal conductivity in thin films for the case of amorphous silicon can be attributable to two factors. Firstly, the limited resolution of thermal measurements (50 nm) and, secondly, the challenges associated with measuring small thermal conductivities that are characteristic of amorphous materials. However, it can be conjectured that disorder could potentially cause size effects that manifest themselves at smaller thicknesses. In order to achieve a more precise understanding of these phenomena, first-principles molecular dynamics (FPMD) can be employed. This approach provides a quantitative atomic-scale description of materials properties, including anharmonicity effects a significant impacting thermal conduction. In this presentation, the methodology for obtaining the thermal conductivity of disordered materials described by FPMD using thermal transients (via the approach-to-equilibrium AEMD method) for a range of amorphous materials will be detailed. The physical origins of the size dependence of thermal conductivity for different materials will be discussed by highlighting the quantitative agreement with experimental data for bulk sizes. The observed changes in thermal conductivity, measured over short lengths, can be explained by invoking current theories of thermal transport in disordered materials. Finally, the impact of these effects on applications based on amorphous materials, such as phase-change devices, will be considered.