A theoretical model allowing us to calculate the size distribution of neutral and charged micellar systems from molecular parameters of the aggregating monomers is given. The derivation includes the range of oligomeric species between the monomer and the average-size micelle. An loss of entropy, resulting-from the reduction in allowed orientations of solvent H-bonds in the neighborhood of nonhydrophilic parts of a solute molecule or aggregate, is introduced as the main factor influencing the size distribution of the aggregates. Equations describing the distribution in terms of known molecular quantities and only a few free parameters are given. Numerically evaluated examples are discussed. The predictions of the model are in very good agreement with the observed aggregation behavior for several classes of solute molecules in aqueous medium.