Ion selectivity by cylindrical pores has been investigated using the primitive model of electrolytes and the classical density functional theory. It is found that a neutral pore always exhibits the preferential adsorption of large ions, regardless of the pore size and the bulk electrolyte composition. For a charged nanopore, however, the pore selectivity depends not only on the ion size but also on the pore radius as well as the surface electrical potential. While a nanopore prefers adsorption of small counterions when its radius is sufficiently large, it can be large-ion selective when its size becomes comparable to the ion diameters.