In this article we present a study on the influence of the size of mesoscopic gold nanostructures on silicon (Si) with well-defined geometry on their reactivity towards hydrogen evolution and carbon dioxide reduction. The onset potential for these two reactions decreases by 170 mV with structure size decreasing from 1400 nm to 75 nm, whereby hydrogen evolution is accelerated somewhat more than carbon dioxide reduction. This effect is independent of the doping type and level of the substrate, i.e. it occurred for moderately doped p-type Si (p-Si) under illumination, as well as on n-type (n-Si) and degenerately doped p-type Si (p(++)-Si) in the dark. Therefore, the increase in reactivity is not influenced by the metal/semiconductor contact which changes from Schottky-type for p-Si to ohmic for p(++)-Si. Due to the relatively large structure size, quantum confinement and a change in surface defect density do not play a role. We argue that the role of the metal structure edges, which increases with decreasing structure size, is decisive. At the edges, the electrostatic charge and potential distribution as well as the chemical environment are different compared to the conditions at the interior surface. At present it remains unclear whether the reactivity enhancement is predominantly due to an electrostatic effect or a chemical, bifunctional mechanism. (C) 2018 Elsevier Ltd. All rights reserved.