Meal size is a major determinant of energy intake and an important phenotype in animal models of obesity and in human eating disorders. Successful analysis of the controls of meal size is a fundamental goal of the science of ingestion. This paper proposes a new classification of the controls of meal size based on an unambiguous physical criterion. The criterion is food stimuli contacting preabsorptive receptors along the surface of the gut from the tip of the tongue to the end of the small intestine. Direct controls depend upon such contact. Indirect controls, e.g., rhythmic metabolic, cognitive, etc., do not have such contact. Instead, indirect controls change meal size by modulating the potency of direct controls. A method of measuring the potency of direct and indirect controls is described. The classification is unambiguous, comprehensive, and explicates the functional relationship between indirect and direct controls. Because the method of measurement is quantitative, this classification is heuristic foe mechanistic research and provides a common theoretical framework for diverse investigators interested in different aspects of the controls of eating and meal size.