The ability of mean-field-based theories to extract pore size information from adsorption isotherms of microporous materials is assessed. These theories represent natural and simple extensions of the Kelvin equation commonly applied in mesopore analysis. Adsorption behavior for a number of model spherical micropores is accurately determined using statistical mechanics. Using these results, pore sizes are calculated via a prototypical mean-field method proposed by Horvath and Kazawoe and compared to the true values. Although moderately accurate, it is found that several key underlying assumptions in the method limit its quality. Most notably, simple mean-field approaches calculate polydisperse pore size distributions for systems which are in fact monodisperse in pore size. Improvements to the methodology are discussed and explored.