Establishing the criteria for the enterprises classification by their size i.e. their classification into micro, small, medium and large ones (SME) is of particular importance for their business operations and the development of an economic branch, a region and/or the national economy as a whole. Such criteria researching stems from the achievements and needs of science legislation and business practice. The legal and economic arguments are presented in favor of classifying the enterprises into micro, medium and large ones. In this paper three interrelated problems are analyzed, referring to the enterprises size determination: how should their size be presented; how should their size be measured; where to set the separation line? Also, four criteria for classifying the enterprises by their size are analyzed: the quantitative(statistical); the qualitative (functional); the institutional the mixed (combined). It is concluded in the paper that research and an appropriate selection of criteria for the enterprises classification by their size into micro, small, medium and large ones is the necessity and the result of the economic and legal science, as well as the necessity of the legislation and the business practice. Also, the enterprises classification by their size is important for setting the position, the role and importance of each group of enterprises - micro, small, medium and large ones. Without objectively set criteria for the enterprises classification by their size there is no reliable statistical basis and qualitative scientific and professional research. Also, without qualitative research and relevant statistical data there is no qualitative and reliable legislation, as a legal assumption to realize the strategy and policy on SME.