Adsorption of methane was measured on BPL-6 activated carbon, and simulated using the Grand Canonical Monte Carlo method in 40 slit-shaped pores ranging in width from from 0.63 to 5.72 nm (1.65-15 methane molecular diameters). The simulations were used, in combination with a single experimental isotherm at 308 K, to extract a pore size distribution (PSD) for the carbon. This PSD was then used, in combination with higher-temperature simulations, to predict methane adsorption on the same adsorbent at two higher temperatures. The predicted isotherms show excellent agreement with experiment at 333 and 373 K for pressures of up to 3 MPa. We demonstrate that the sensitivity of the PSD to characterization using a supercritical adsorbate such as methane is bounded by a limiting pore width, which is a function of pressure, temperature, and the adsorbate used. Above this limiting pore width, adsorption maybe used to characterize porous solids instead by their surface area. At 308 K, the PSD of micropores can be effectively characterized using methane adsorption at pressures up to 3 MPa. An implementation of our method is available on the World Wide Web.