The electrical double layer (EDL) formation near a charged surface is a very important phenomenon for various physical, chemical, and biological systems. As an example, transport of ionic species through nanometer-scale pores can be controlled by the EDL structure inside the pores. Since ion size is the most important property when dealing with the physical chemistry of electrolyte solutions, the effect of this property on EDL formation inside a nanopore is of specific focus in this study. Monte Carlo simulations were performed to provide insights into the phenomena relating the pore size to the properties of ionic species in the EDL formation inside nanopores. Three competitive factors can explain the fundamental mechanisms: displacement due to ion-size, effect of charge asymmetry, and occurrence of EDL overlapping. Carbon-based membranes synthesized at Oak Ridge National Laboratory constitute very attractive materials because of their highly ordered nanopores and good electrical conductivity. A single symmetric electrolyte (i.e., sodium chloride) is used to prove the ability of ion-selective transport by using nanoporous carbon-based membranes. A mixture of electrolytes with a marked difference in the diameter of hydrated ions is simulated to examine the effect of ion size on the EDL formation and ion transport through nanoporous carbon membranes. In addition, electrochemical measurements via cyclic voltammetry are used to study the relationship between pore size of nanoporous carbon electrodes and ion size of electrolytes. The combination of experimental data and molecular simulations provides a realistic approach to identifying the mechanisms of EDL formation inside nanopores and its effects on ion sorption and transport within nanostructured materials.