Electromagnetically induced transparency (EIT) effect has been studied using an extremely thin cell (ETC) with the thickness of an Rb vapor column of the order of light wavelength $\lambda \left(780\mathrm{nm}\right)$ and varying in the range of $0.5\lambda –2.5\lambda$ . Λ-systems on the $D_2$ line of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ have been studied experimentally. Along with EIT resonance, we study the peculiarities of velocity-selective optical pumping/saturation (VSOP) resonances, which accompany the EIT resonance and, as a rule, are spectrally broader. It is demonstrated that size-conditioned strongly anisotropic contribution of atoms with different velocities in an ETC causes several dramatic differences of the EIT and VSOP resonances formation in the ETC as compared with an ordinary $1–10\mathrm{cm}$ long cell. Particularly, in the case of the ETC, the EIT linewidth and contrast dramatically depend on the coupling laser detuning from the exact atomic transition. A theoretical model taking into account the peculiarities of transmission spectra when $L=n\lambda$ and $L=(2n+1)\lambda ∕2$ (n is an integer) has been developed. The experimental transmission spectra are well described by the theoretical model developed. The possibility of EIT resonance formation when atomic column thickness is of the order of $L=0.5\lambda$ and less is theoretically predicted.