A national survey of college faculty provided data for an analysis of class size norms in postsecondary institutions, and the institutional and academic variables associated with class size differences. Institutional characteristics considered include public versus private control, Carnegie classification, and institution size. Course academic characteristics include academic discipline, classified according to the A. Biglan (1973) model, instructional level, and principal method of instruction. Only lower division courses were considered, and the one percent of classes at each extreme of size were eliminated. Results showed class size to be a function of both institutional and course characteristics. Classes demanding active participation and those at smaller, liberal arts, and two-year institutions were smaller. Classes were also smaller in applied fields and fields focusing on living organisms. Other factors (e.g., private/public control, discipline paradigms) explained little variance. A resulting series of class size distributions is presented. (Contains 4 tables of data and 15 references.) (MSE)