Cu2O is a p-type semiconductor which attracts much attention for application in photovoltaics, photocatalysis and solar water splitting. However, Cu2O is not intrinsically stable under illumination in aqueous solutions, and the edge of the valence band is not positive enough to provide sufficient overpotential for water oxidation. The stability and band edge position of nanostructured materials depend on crystallite size. In this paper, we describe a new strategy to vary the size of Cu2O nanoparticles using mesoporous silica supports. First, CuO nanoparticles were obtained via impregnation-drying-heating. The size of the nanoparticles was tuned by varying either the concentration of Cu precursor or the pore diameter of the supporting silica. Subsequently, the CuO was converted to Cu2O without particle growth by gas-phase reduction with carbon monoxide. The visible light absorption of these nanoparticles depended on the copper oxide phase and crystallite size, leading to a direct band gap energy of 2.60 eV for 2 nm Cu2O nanoparticles compared to 1.94 eV for macrocrystalline Cu2O. Our results highlight a new synthesis strategy for the preparation of metal-oxide nanoparticles with controlled sizes of 2–15 nm that are not directly accessible by alternative synthesis techniques. The as-obtained 15 nm Cu2O nanoparticles were used for H2 evolution in a water-methanol mixture, the photocatalyst gave a H2 evolution rate of 11.5 × 10−3 μmol min−1 which corresponded to an internal quantum efficiency of 15.8% and an overall quantum efficiency of 3.5% for light between 310 and 710 nm. Finally, the nanoparticles were stable during three hours of light illumination.