A new method for sizing the diameter of cylindrical mesopores is presented in this paper. The effects of the pore dimension and the cylindrical shape on adsorption in mesopores are accounted for by using the concepts of enhanced pore pressure and enhanced surface layering. In conjunction with the convectional Kelvin equation, this treatment is capable of modeling the capillary condensation process in mesopores. The model is tested against adsorption data of benzene and nitrogen onto MCM-41 samples. The isotherm model fitting is satisfactory. The sizing of the pore in MCM-41 using this approach does not suffer from the pore size underestimation problem experienced with the traditional Kelvin equation.