The alkaline hydrogen evolution reaction (HER) plays a pivotal role in realizing a H₂-based circular economy. However, it is a topic of ongoing debate because of its complexity. Here, we unveil a unique volcano-type size-dependent activity trend of archetypical Ru-based alkaline HER catalysts and demonstrate that this trend is dictated by the interplay of geometric and electronic effects. Size-controlled Ru nanoparticles (NPs) from 0.78 to 3.31 nm were synthesized, and they exhibited volcanic size-dependence of specific activity and reaction kinetics, with 1.38 nm Ru NPs showing the highest activity and lowest Tafel slope. The large variation in the Tafel slope of Ru NP catalysts from 29 to 109 mV dec^–1 suggests that tuning of Ru NP size can vary the rate-determining step in the order of the Heyrovský (0.79 nm), Tafel (1.38 nm), and Volmer (3.31 nm) steps. The specific activity of 1.38 nm Ru NPs is 5.0, 2.6, and 1.2 times higher than those of 3.31 nm Ru NPs and commercial Pt/C and Ru/C catalysts, respectively. Atomic-level geometric structure analysis and density functional theory calculations revealed that excellent activity of 1.38 nm Ru NPs correlates with their abundance in edge sites, which show optimum H* binding energy and elementary step energetics. A significant decline in the intrinsic activity of the alkaline HER was observed in the subnanometer Ru NPs, which could be associated with suppressed H* chemisorption due to enhanced surface oxidation and amorphous surface nature in this size regime. Overall, the intrinsic activity trend of Ru NPs is governed by both the geometric fraction of active edge sites and suppression of H* intermediate chemisorption in the subnanometer regime.