Understanding and optimizing the effects of edge states and nanoflake dimensions on the photon harvesting efficiency in ultrathin transition-metal dichalcogenide (TMD) semiconductor photoelectrodes is critical to assessing their practical viability for solar energy conversion. We present herein a novel filtration-based separation approach to systematically vary the TMD nanoflake dimensions and edge density of solution processed large-area multiflake WSe2 photocathodes: Photo electrochemical measurements in both aqueous electrolyte (for water reduction) and a sacrificial redox system, together with a continuum-based charge transport model, reveal the role of the edge sites and the effects of the flake size on the light harvesting, charge transport, and recombination. A selective passivation technique using atomic layer deposition is developed to address detrimental recombination at flake edges. Edge-passivated WSe2 films prepared. with the smallest flakes (similar to 150 nm width, 9 nm thickness) demonstrate an internal quantum yield of 60% (similar to bulk single-crystal results). An optimized (1 sun) photocurtent density of 164 mA cm(-2) is achieved with 18-nm-thick flakes (700 nm width), despite transmitting similar to 80% of the accessible photons: Overall, these results represent a new benchmark in the performance of solution-processed TMDs and suggest routes for their development into large-area low-cost solar energy conversion devices.