Herein we report our efforts to control the size of colloidal Ni nanoparticles (NiNPs) via a seed-mediated approach and to produce supported Ni catalysts that are sinter-resistant. NiNPs are prepared using a mild capping ligand and an external reducing agent at 90 °C to obtain seeds of 3-4 nm, followed by NiNP growth at 220 °C to vary the final size up to 8 nm. These NiNPs were either introduced onto high surface area silica via direct deposition in organic solvents, or encapsulated in mesoporous silica. Encapsulation was determined to be the most promising approach to support the particles. A range of such encapsulated NiNP catalysts were evaluated for CO2 hydrogenation between 200-400 °C: they were comparably active as catalysts reported in the literature, and thermally stable and sinter-resistant for 70 h at 350 °C. Nevertheless, it was found that the Ni phase was redistributed throughout the mesoporous silica network, resulting in Ni catalysts with nearly identical particle size of 4-5 nm, determined by the size of the support pores, after a combination of oxidative and reductive pretreatments. Our approach provides a route to obtain Ni@SiO2 catalysts with a narrow particle size distribution from a range of colloidal NiNP precursors.