While hot carrier generation from surface plasmon decay at the surface of a nanostructured metal offers a distinctive concept for boosting photoelectrocatalytic reactions, the nature of the plasmonic hot hole transfer based on the sizes of metallic nanomaterials has not been investigated in depth experimentally. Here, we report direct photoelectrochemical (PEC) experimental proof that the injection of plasmonic hot holes depends on the size of the metallic nanostructures. PEC results clearly indicate that a plasmonic template with smaller Au nanoprisms exhibits higher external and internal quantum efficiencies, leading to a significant enhancement of both oxygen evolution and hydrogen evolution reactions. We verified that these outcomes stemmed from the enhanced hot hole generation with higher energy and transfer efficiency driven by enhanced field confinement. These findings provide a facile strategy by which futuristic photocatalysis and solar energy conversion applications based on plasmonic hot holes can be expedited.