Transition metal dichalcogenides (TMDs) are efficient electrodes for photoelectrochemical solar energy conversion to electricity and chemical fuels. One approach to produce large-area thin film photoelectrodes is to exfoliate single or multiple TMD layers from high quality bulk samples. However, exfoliated TMD thin films exhibit lower photocurrent collection efficiencies than bulk electrodes. Here we use a single-nanoflake photoelectrochemical microscopy approach to determine how variations in nanoflake area, thickness, and surface structural features contribute to the lower photocurrent collection efficiency in n-MoSe2/I-,I3-/Pt solar cells. Photocurrent efficiency increases with nanoflake area, but is independent of nanoflake thickness over the range of 40-400 nm. Surprisingly, there is a small, "champion" population whose photocurrent efficiency values are nearly equivalent to or exceed the bulk crystal. This observation, which is hidden in ensemble-average measurements, shows that exfoliated nanoflakes can achieve similar photocurrent collection efficiencies to bulk crystals. There is a also a large, "spectator" population that is mostly responsible for the overall lower photocurrent efficiency compared to the bulk crystal. Photocurrent mapping showed that charge carrier recombination near perimeter edges is more significant than at interior steps. Our results highlight nanoflake properties that should be considered and optimized for high performance exfoliated TMD photoelectrodes.