The hydrodynamic solutions based on Maxwell's boundary conditions include an empirical slip coefficient (SC), which depends on properties of the adsorbate and adsorbent. Existing kinetic theory derivations of the SC are usually formulated for half-space flow and do not include finite-size effects, which dominate the flow in nanopores. We present an expression for the SC applicable to flow in nanoscale pores, which has been verified by nonequilibrium molecular-dynamics simulation. Our results show that the slip coefficient depends strongly on the pore width for small pores tending to a constant value for pores of width >20 molecular diameters for our systems, in contrast to the linear scaling predicted by Maxwell's theory of slip.