The effect of ionic size on both the degree of dissociation of the surfactant shell of a spherical reverse micelle and the spatial distribution of ionic species inside is examined on the basis of a modified Poisson-Boltzmann equation and a local ideal mixing rule. The dissociation of surfactant molecules is the consequence of the competition between the entropic effect of counterions in the aqueous core, the osmotic effect of the surfactants on the micelle shell, and the effect of electrical interactions. We show that the ion distribution close to the surfactant shell can be altered remarkably, and the saturation adsorption concentration varies with dissociation conditions. The free energy associated with the dissociation of surfactant molecules is studied taking the steric effect of ionic species into account. The presence of a neutral particle in a reverse micelle is found to have a negative effect on the degree of dissociation of the surfactant shell.