Two-photon Dicke-narrowing selective reflection (TDNSR) spectroscopy is studied in a system of Λ-type three-level atoms confined between two dielectric walls. TDNSR lines are investigated for the cases of different thicknesses of the vapor film and different strengths of the pump field. Because of de-excited behavior induced by atom-wall collision, effect of slow atoms in a thin atomic vapor film and Doppler-free configuration, sub-Doppler lines can be obtained in many cases. It is also found that TDNSR lines change periodically with the ratio of L/λ (L is the thickness of the atomic film and λ is the wavelength of the probe field). TDNSR disappears for L=2mλ/4 (m is an integer) and revives a typical dispersion shape for 2mλ/4