During its cell cycle, the rod-shaped fission yeast S. pombe cell grows to ~14μm in length and ~160μm2 in surface area before entering mitosis. Cell size homeostasis studies suggest that fission yeast is capable of sensing its own size, although the underlying mechanism remains unclear. The membrane-associated protein kinase Cdr2 is a candidate sizer that regulates the G2/M transition. Indeed, Cdr2 forms clusters on the plasma membrane, the so-called “nodes,” in a cortical region at midcell and the number of these nodes correlates with cell size. However, what aspect of cell size is measured by Cdr2, and how, remain controversial and unanswered questions. Here, we show that cells lacking Cdr2 exhibit much weakened cell size homeostasis, confirming a key role for Cdr2 in cell size control. Moreover, analysis of cells with different radii suggests that cells sense their surface area instead of their volume or length. This result can be explained by a predictive mathematical model where the ratio between total Cdr2 nodal levels and the area of the nodal region, the Cdr2 nodal density, acts as a sensor of cell surface area. Overall, our approach reveals a mechanistic basis for cell size sensing in fission yeast.