The increasing CO2 concentration in the seawater can potentially stimulate photosynthesis of phytoplankton, as the low diffusivity of CO2 can limit its supply to phytoplankton. Although it is often considered that large phytoplankton benefit most from increasing CO2, we analyzed historical experimental data and found that the growth rate response (GRR, relative increase of growth rate) of phytoplankton maximizes at medium cell size. In this study, we establish a numerical model representing CO2 concentrating mechanism (CCM) of phytoplankton cells, and simulate GRR of different-size phytoplankton under seawater CO2 concentration from 10 μM (LC) to 20 μM (HC). The model includes two pathways of inorganic carbon supply to phytoplankton, including CO2 diffusion and energy-consuming bicarbonate (HCO3-) uptake. The model also reveals an optimal cell size for GRR. When cell diameter is very small, CO2 diffusion is sufficient to support cell growth under both LC and HC and GRR is 0. With increasing cell size, cell under LC needs energy to take up HCO3-, and GRR increases until reaching a maximum at a cell diameter when cell under HC also starts to need HCO3-. Larger than that, phytoplankton under both LC and HC cost energy to take up HCO3-, CO2 diffusion becomes less important with increasing cell size, and GRR declines. Numerical experiments further show that the predicted optimal cell diameter is the most sensitive to rates of cell carbon requirement and CO2 leakage. Future work should explore the impact of various processes of the model and integrate with laboratory experiments.