The article presents the analysis of economic size of private farms in Poland in the years 2004-2012. The average farm in the province was adopted as the research unit. The main aim of the study was to assess the regional diversity of economic size of farms and its changes over time. However, changing the definition of a farm has caused an additional problem, due to the heterogeneity of data. Therefore, in the first part of the research, for each region and years 2004-2009, number of individual farms that have met the current definition was estimated. The next step was to evaluate the economic size of average farm. The economic size of farm determines production capacity of the farm, expressed as its potential income. Based on the economic size of the farm, estimated for each of the years of research, the rankings of regions were created. High concordance of rankings suggested that the division of provinces into groups with similar levels of development of agriculture would be stable over time. This assumption was verified by standard deviation method.