The paper presents data on phase equilibria in Cu-S, Ag-S, Zn-S, Cd-S, Hg-S, Sn-S, and Pb-S systems and generalizes experimental results on the crystal and electronic structure of twenty polymorphic phases of coarse- and nanocrystalline semiconductor sulfides of copper, silver, zinc, cadmium, mercury, tin, and lead, including information on the band gap width of the sulfides considered. For the first time, using data on the effective masses of charge carriers and dielectric constants, information on the sizes of excitons in coarse-crystalline sulfides is generalized. Allowing for the sizes of excitons, the size effects observed in the electronic (optical) properties of the above sulfides in the nanocrystalline state are considered. It is shown that a decrease in the size of sulfide nanoparticles can be accompanied by a change in the stoichiometry of sulfides as well as their lattice properties.