We prepared synthetic porous carbons with a three-dimensional interconnected network of ultramicroporous particles; by adjusting the synthesis parameters the size of these particles was systematically varied from 13nm to 1.4μm. Cyclic voltammetry measurements using a cavity microelectrode (CME) were performed in different chloric aqueous electrolyte solutions. The CME allows for very fast measurements (up to 10V/s) of carbon powders without the need of a binder or conductivity additive. Therewith we were able to investigate the accessibility of the ultramicropores and kinetics of the electrochemical double-layer (EDL) charging as a function of particle, inter-particular pore, effective ion and micropore size. We found, that the alkaline cations K+, Na+, Li+ and the anion Cl− can enter micropores down to a size of 0.4nm at slow charging rates independent of the particle size. However, the accessibility is decreased due to kinetic limitations for the cations Na+ and Li+ at higher scan rates, which was not observed for the smaller K+ ion. A simple model is presented, which explains the influence of the particle, inter-particular pore and micropore size on the kinetics of double-layer charging. This model suggests that first of all the carbon particle size is not a limiting factor for fast EDL charging as long as its size is smaller than a few hundred nanometers and secondly inter-particular pores with a size in the range of the particle size or larger are crucial for high-rate applications.