In 2010, the CSO has altered the definition of a farm. As a result of this change, there are some research problems, arising from a significant decrease in the number of farms that meet the new definition. One of them is the impact of this decline on the regional differentiation of economic size (EC) of individual farms. The aim of the paper was to evaluate the impact of this decrease on the regional differentiation of economic size of farms. The analysis covered the year 2010. It is the only year, for which data about the number of farms that meet both the old one and the new definition, are available. Based on CSO data, for both definitions and for each province the economic size of average private farm were estimated. Upon the results of the estimation, the ranking of regions was created. The provinces were also classified and divided into groups. As it was proved in the study, the change in the definition has significantly affected the regional diversification of economic size. Although ranking of provinces was only slightly altered, the change of the definition substantially influenced the results of dividing provinces into groups, regardless of the method of clustering.