Electrical double layer (EDL) formation in nanoporous materials is important to many physicochemical processes of practical significance, including energy storage in super-capacitors and water purification. The effects of ion size and charge of electrolytes confined in charged nanopores are simulated here by Monte Carlo techniques, including Canonical Monte Carlo and Grand Canonical Monte Carlo. Simulation results show that multivalent and monovalent ions present different characteristics under an electrostatic field in nanopores. When the pore size approaches to the dimension of ionic species, EDL overlapping occurs, which limits participation of ions in EDL formation. The distribution of ionic species is also very sensitive to the ion size. Furthermore, charge inversion may occur under certain conditions of high electrolyte concentration and high surface charge. This phenomenon is more evident in the presence of multivalent counterions of a larger diameter. Three competitive factors can explain the fundamental mechanisms behind the EDL behavior: (i) ion displacement due to ion-excluded volume, (ii) asymmetry in ion charge, and (iii) occurrence of EDL overlapping. Besides the modeling work, electrochemical characteristics are revealed by cyclic voltammetry, demonstrating that the relationship between the pore size and ion size can determine the EDL capacitance of nanoporous electrodes. This study concludes that the consideration of ion size, in both experimental and theoretical studies, is essential to our understanding of the EDL behavior of nanoporous materials, as well as to applications of practical significance.