A three-dimensional Eulerian–Lagrangian fluid dynamics (CFD) model was developed to simulate the gas particulate flow inside cyclones with different sizes. Cyclones of different sizes, were used which named as cyclones I, II, and III. Cyclone I was considered as the biggest and cyclone III was considered as the smallest cyclone. The effects of cyclone size and inlet velocity on hydrodynamics behavior and performance parameters including cut-off diameter and pressure drop were investigated. The renormalization group (RNG) k–ɛ model and Reynolds stress model (RSM) were used to study the effect of turbulent modeling. Particle trajectories were calculated via discrete phase model (DPM). The velocity fluctuations were simulated with discrete random walk (DRW) model to study the turbulent dispersion of particles. The cut-off size and pressure drop were increased with increasing the cyclone size. The RSM predicted the cut-off diameter very well with the deviations of 2.3%, 3.4%, and 3.6% of the experimental data, for cyclones I, II, and III, respectively. CFD model was developed using Fluent code to simulate the gas particulate flow inside cyclone. The simulation results also confirmed the applicability of CFD modeling with RSM as a promising tool to study the cyclone size effect on performance parameters.