In this paper, we study the dynamics of a nutrient–phytoplankton–zooplankton model incorporating the factor of plankton body size in deterministic and stochastic environments. The value of this paper lies in two aspects: Mathematically, for the model without stochastic environmental fluctuations, we give the existence of boundary and positive equilibria, show the local and global stability of these equilibria, and prove the existence of Hopf bifurcation. For the model with stochastic environmental fluctuations, we provide the results related to the stochastic extinction and persistence in the mean, as well as the existence of ergodic stationary distribution. Ecologically, via numerical simulations, we find that plankton body size plays a key role in regulating the dynamics of interacting plankton in the deterministic and stochastic environments. It is worth emphasizing that the increase of phytoplankton cell size can stabilize the model by excluding the periodic solution induced by nutrient removal rate, while the increase of zooplankton body size can remain such periodic phenomena. Additionally, it should be noted that the very small phytoplankton cell size or zooplankton body size has a negative effect on the plankton growth in the stochastic fluctuation environments. These results may help to understand the complex dynamics of plankton models.