The effects of ion size on the current efficiency (CE) of charged membranes with narrow pores are studied theoretically. The CE is a measure of the membrane permselectivity defined as the ratio between the counterion flux and the sum of the counterion and coion fluxes when an electric potential difference is applied between the two solutions bathing the membrane. It is studied here as a function of two relevant experimental parameters: the ratio between the ionic radius and the pore radius, and the ratio between the external salt concentration and the membrane fixed charge concentration. The ratio of the CE values corresponding to the point and finite size ions is also calculated as a function of the above two parameters. Ion size effects are introduced in the ionic diffusion coefficients (by means of the Renkin function) as well as in the ionic concentration profiles. It is shown that a significantly higher coion exclusion from the membrane and thus a higher CE is obtained for finite size ions compared to the case of point ions.