Chalcogenide-based nanostructured phase-change materials (PCMs) are considered promising building blocks for non-volatile memory due to their high write and read speeds, high data-storage density, and low power consumption. Top-down fabrication of PCM nanoparticles (NPs), however, often results in damage and deterioration of their useful properties. Gas-phase condensation based on magnetron sputtering offers an attractive and straightforward solution to continuously down-scale the PCMs into sub-lithographic sizes. Here we unprecedentedly present the size dependence of crystallization for Ge2Sb2Te5 (GST) NPs, whose production is currently highly challenging for chemical synthesis or top-down fabrication. Both amorphous and crystalline NPs ha...