Abstract: Cell size of phytoplankton is regarded as a result of environmental adaptation and plays a critical role in the regulation of photosynthesis; however, few information on the response of photosynthetic activity to different cell sizes in the same species could be obtained. This study investigated the effects of cell size on photosynthesis in Synechocystis sp. PCC 6803. The results indicated that smaller cells, due to the larger specific surface area, exhibited higher concentrations of chlorophyll a per unit volume and a higher specific absorption coefficient for light energy. However, the limited chlorophyll storage and constrained reaction center capacity reduced light energy utilization efficiency under high light intensity conditions. In contrast, larger cells encountered limitations in material exchange and light capture, demonstrated higher photosynthetic efficiency and better adaptability under high light intensity. This was achieved by optimizing the absorption, utilization, and distribution of light energy. Larger cells exhibited superior performance in several key metrics, including the maximum photochemical quantum yield (F_v/F_m), the actual quantum yield of photosystem II (ΦPSII), the maximum photosynthetic rate (ETR_max), the number of reaction centers per unit excitation area (RC/CS_O), photosynthetic rate per unit chlorophyll a, and greater respiratory stability. This study elucidated the trade-off between cell size and photosynthetic activity, confirming the advantages of smaller cells in rapid light capture and emphasizing the superior high light adaptability of larger cells. These findings provide new insights into the photosynthetic adaptation mechanisms of Cyanobacteria.