The adsorption of pure pentane (C-5) isomers and their ternary mixture is simulated in a series of carbon nanoslits. With decreasing nanoslit pore size, shape selective adsorption first occurs in the order of nC(5) >= iC(5) > neoC(5) due to the configurational entropy effect, then inverse-shape selective adsorption occurs in the order of nC(5) < iC(5) e neoC(5) due to the area entropy effect, and finally no adsorption occurs. The entropy effects lead to a large adsorptive separation among the C5 isomers from their mixture. Similar behavior has been observed from the simulation of C5 adsorption in carbon nanotubes with variation in pore size. These results reveal that pore size rather than geometry determines the shape and inverse-shape selective adsorption of alkane isomers in nanopores.