Plasmonic-metal/semiconductor heterostructures represent promising photocatalytic constructs for solar-to-fuel energy conversion, but detailed knowledge of the structure-function relationships governing their activity remains elusive. Here, we monitor plasmon-mediated electron transfer (PMET) across Au-TiO₂ Schottky junctions at the single-nanoparticle level to clarify the role of metal nanocrystal size in plasmonic photochemistry. Size-dependent photovoltage studies of PMET in Au/TiO₂ photoelectrodes reveal that the reduction potential of hot electrons within the TiO₂ conduction band is not solely determined by the absolute size of Au nanoparticles, but by the relative size of Au to TiO₂. With this new insight, we tailored the physical dimensions of Au/TiO₂ photocatalysts to efficiently harvest hot electrons for improved plasmon-driven hydrogen evolution from water. Taken together, these studies elucidate the role of metal nanocrystal size in plasmonic photocatalysis and establish general guidelines for the rational design of plasmonic-metal/semiconductor assemblies that can effectively exploit hot carriers in photochemical reactions.