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The effect of electromagnetically-induced transparency in the ladder system of 5S1/2-5P3/2-5D5/2 levels of Rb atoms is studied. The effect of spectral narrowing of the dark resonance (DR) is shown depending on the density of atomic vapor and the thickness L of the spectroscopic cell, which contains Rb atomic vapor. The thickness L varies from 390 nm to 4 mm, the atomic density N increased to ~ 10^16 cm^-3, and intense coupling and weak probe radiation were used. The maximum effect, namely the 22-fold spectral narrowing of DR, was achieved in a cell with a thickness of L = 4 mm. With decreasing of the thickness L, the effect of spectral narrowing becomes weaker: for example, at L = 2 μm, a 2.4-fold spectral narrowing of DR occurs. The spectral narrowing of DR was practically absent at L = 0.8 μm, and with a further decrease to L = 0.4 μm with increasing of atomic vapor density, spectral broadening of DR began to occur. In almost all cases, at moderate atomic densities and high intensities of the coupling radiation, ~ 100% DR contrast was achieved. The effect of spectral narrowing and broadening of DR is explained.