This study presents the effects of particle size on the monolayer structure of silica nanoparticles in the diameter range between 10 and 100 nm via a wet-coating process. Suspensions of silica nanoparticles coated on a solid substrate by a bar coater are dried to give a monolayer. For the comprehension of dominant factors in the formation process, the experimental results are compared with the results obtained by numerical simulations. Dense monolayers can be obtained in the diameter range of silica nanoparticles from 10 to 100 nm via the wet-coating process; however, the monolayers of the nanoparticles, the diameter of which is 11 nm, cannot be obtained at the coverage over 0.7. The pair correlation function of the monolayer shows that the 11-nm nanoparticles are arranged with the inter-particle separation of about 1-2 nm. A feasible model for the formation process suggests that the hydration force between silica surfaces retains the nanoparticles with a constant separation.