Structural properties of porous silicon are investigated theoretically. Distribution functions of pore sizes were introduced and expressions for the surface and volume porosities of the material are obtained. The suggested models allow one to calculate the specific surface area and its dependence on porosity. A model for macroporous silicon with a nanoporous layer on the pore walls is suggested also. An attempt is done to describe analytically the observed phenomenon of the specific surface area decrease at higher porosities.