Phytoplankton size distributions significantly impact structure and functioning of aquatic ecosystems by affecting higher trophic levels and key biogeochemical cycles. Correctly predicting phytoplankton size distributions under different conditions remains elusive. We used literature-derived allometric relationships between key parameters of nutrient uptake and growth and cell size to develop a model of size selection in phytoplankton. The model uses the adaptive dynamics approach that includes ecological interactions to determine evolutionarily stable strategies or other selective outcomes with respect to size. We found that the identity of a limiting nutrient may influence the direction of size selection: nitrogen limitation in diatoms lead to the selection of a large cell size, while phosphorus limitation favored small cells. These results can be explained by different allometries for the two nutrients arising from physiological differences in acquisition and utilization of these nutrients.