The adsorption—for separation, storage and transportation—of methane, hydrogen and their mixture is important for a sustainable energy consumption in present-day society. Graphene derivatives have proven to be very promising for such an application, yet for a good design a better understanding of the optimal pore size is needed. In this work, grand canonical Monte Carlo simulations, employing Improved Lennard–Jones potentials, are performed to determine the ideal interlayer distance for a slit-shaped graphene pore in a large pressure range. A detailed study of the adsorption behavior of methane, hydrogen and their equimolar mixture in different sizes of graphene pores is obtained through calculation of absolute and excess adsorption isotherms, isosteric heats and the selectivity. Moreover, a molecular picture is provided through z-density profiles at low and high pressure. It is found that an interlayer distance of about twice the van der Waals distance of the adsorbate is recommended to enhance the adsorbing ability. Furthermore, the graphene structures with slit-shaped pores were found to be very capable of adsorbing methane and separating methane from hydrogen in a mixture at reasonable working conditions (300 K and well below 15 atm).