Oriented mesostructured surfactant-silica nanocomposite thin films with a 2D hexagonal mesostructure of cylindrical micelles were prepared by evaporation-induced self-assembly using two different nonionic Brij surfactants and studied by small-angle X-ray scattering in symmetric reflection (SRSAXS) and grazing incidence (GISAXS) geometries. A novel SRSAXS evaluation approach was applied that allowed a good fitting of the SRSAXS data over almost the whole range of scattering vectors. Aside from the cylinder radius and the lattice parameter, the approach provided accurate values for the polydispersity of the micelles, lattice distortions, and preferred orientation. These analyses revealed a significant rise of the micelle radius and accordingly the lattice parameter upon an increase in the ratio surfactant/SiO2, attributable to a decrease in the solubilization of the poly(ethylene oxide) (PEO) chains by water, in agreement with Monte Carlo simulations. Furthermore, the SRSAXS analysis was successfully applied to the corresponding mesoporous films for the determination of pore sizes.