Using the two-dimensional coupled-wave theory,the diffraction properties of the 90-degree readout geometry of the finite size planar holographic grating which recorded by short-wavelength,but reconstructed by telecommunication long-wavelength were investigated. The effects of the grating geometrical size to the diffraction efficiency and to the Bragg selectivity of the grating were studied. It was shown that,as the horizontal direction and vertical direction sizes of the grating increase,the diffraction efficiency of holographic grating increased. When the horizontal and vertical direction sizes were changed while their products contain constant,the diffraction efficiency contain constant. The Bragg selectivity increased with the geometrical size of the grating increasing.