MgH2 is a promising material for reversible solid-state hydrogen storage. It is known that particle size can have a strong impact on hydrogen dynamics and sorption characteristics, but more detailed insight has been hampered by the great challenge to prepare small and well-defined particles and study their hydrogen storage properties upon cycling. The preparation of MgH2 nanoparticles supported on high surface area carbon aerogels with pore sizes varying from 6-20 nm is reported. Two distinctly different MgH2 particle populations are observed: X-ray diffraction invisible nanoparticles with sizes below 20 nm, and larger, crystalline, MgH2 particles. They release hydrogen at temperatures 140 degrees C lower than bulk MgH2. The size-dependent hydrogen kinetics is for the first time corroborated by intrinsic hydrogen dynamics data obtained by solid state H-1 NMR. Fast cycling is possible (80% of the capacity absorbed within 15 min at 18 bar and 300 degrees C), without a change in the hydrogen sorption properties, showing that the growth of the nanoparticles is effectively prevented by the carbon support. A clear correlation is found between the hydrogen desorption temperature and the size of the MgH2 nanoparticles. This illustrates the potential of the use of supported nanoparticles for fast, reversible, and stable hydrogen cycling.