Finality of some theoretical notions is suggested to be tested by the developmental state of the following three characteristics of them systemic complexity, level of generalization, object-boundness. Logical psychological analysis of the notion of size has been performed with the view to reveal the genetically primary relation from which the constitution of the notion starts, as well as the operations instrumental in establishing this relation. A diagnostical procedure is described for estimation of the state of the notion of size in younger schoolchildren.