The transfer of heat from a plasmonic nanoparticle to its water environment has numerous applications in the fields of solar energy conversion and photothermal therapies. Here, we use nonequilibrium molecular dynamics to investigate the size dependence of the interfacial thermal conductance of gold nanoparticles immersed in water and with tunable wettability. The interfacial thermal conductance is found to increase when the nanoparticle size decreases. We rationalize such a behavior with a generalized acoustic model, where the interfacial bonding decreases with the nanoparticle size. The analysis of the interfacial thermal spectrum reveals the importance of the low frequency peak of the nanoparticle spectrum as it matches relatively well the oxygen peak in the vibrational spectrum. However, by reducing the nanoparticle size, the low frequency peak is exacerbated, explaining the enhanced heat transfer observed for small nanoparticles. Finally, we assess the accuracy of the continuum heat transfer equations to describe the thermal relaxation of small nanoparticles with initial high temperatures. We show that, before the nanoparticle loses its integrity, the continuum model succeeds in describing with small percentage deviations the molecular dynamics data. This work brings a simple methodology to understand, beyond the plasmonic nanoparticles, thermal boundary conductance between a nanoparticle and its environment.