Nanosilica is a new material that has been recently used in the construction industry on a mass scale, featuring several advantages and limitations. Despite the extensive research about the nanosilica effect on cement-based materials, so far there is no consensus regarding the optimal nanosilica dosage that further enhances the mechanical performance and durability of mortar and concrete. With the aim of solving this issue, this paper reports on the optimal nanosilica dosage in normal-strength mortar and concrete regarding their mechanical and durability properties. Experiments on fresh (setting time and flow) and hardened state (compressive strength, tensile strength and chloride migration tests) were carried out on mortar and concrete using two silica nanoparticles of 264 nm and 36 nm average size. As a result, it was found that mortar and concrete using 1.5 wt.% to 2.0 wt.% doses of nanosilica showed the best performance for the mechanical and durability properties. Coarser nanoparticles (264 nm) provide mortar and concrete with enhanced mechanical and durability properties compared to the finer nanoparticles (36 nm). Finally it was found that nanoparticle size had no effect on the optimal nanosilica dosages identified in this study.