This work contains a comparative analysis of the size structure of farms in Poland in three main time frames of the one hundred years since Poland regained statehood, i.e. the interwar period (1918–1939), the period of the People’s Republic of Poland (1945–1989), and the period of the Third Polish Republic (1989–2018). Data sources included statistical yearbooks and literature on the subject. The periods considered in the work were periods of radical changes in the concept of shaping the agricultural sector. In the first period, dominated by the urgent need to rebuild the state and its economy, the development of full-fledged private farms was favoured. In the second period, in the face of changing political and economic conditions, these farms were often forcibly closed down, favouring socialist land ownership. In the third examined period, along with the transformation of the economy, the sector of state-owned agricultural holdings was liquidated. After difficult years of adjusting to the realities of free market economy, agriculture also experienced another change related to Poland’s accession to the EU and functioning under the CAP. However, the research results prove that despite the political and socio-economic changes, and even despite radical political decisions, the farm size structure was characterized by a specific inertia. In the light of statistical data describing the centenary of Independent Poland, one can speak of the sustainability of the size structure of individual farms, demonstrating its high resistance to external factors and stimuli.