When confined to the nanoscale, the glass transition temperature (T-g) of polymer films can deviate substantially from the bulk, i.e., the T-g-confinement effect. Due to ease of processing most studies have focused on the thickness-dependent T-g of thin films, while few have focused on extending investigations beyond thin films to other geometries. As polymers confined to higher geometrical dimensionalities become the enabling material in technologies ranging from drug delivery to plastic electronics to ultrafiltration, a greater understanding of size effects on the T-g is warranted. Here, we investigate the effects of three-dimensional confinement on the T-g of polymer nanoparticles under soft and hard confinement and quantitatively compare our results to those of thin films to explore commonalities or differences between the T-g-confinement effect for polymers confined to different geometries. Via modulated differential scanning calorimetry, we show that T-g decreases with size for polystyrene (PS) nanoparticles suspended in an aqueous solution, in agreement with the corresponding freestanding films. Furthermore, capping of PS nanoparticles with a hard silica shell leads to a size invariant T-g. These results suggest that the free surface is a key factor in T-g reductions of confined polymer, irrespective of geometry.