The importance of high velocity for good particle deposition efficiency in the cold spray process is well known. Small particles achieve high velocities during nozzle acceleration, but also decelerate rapidly in the flow downstream of the bow shock wave. This study examines the effect of particle size on velocity and deposition efficiency in the cold spray process by means of flow modeling and gas-particle dynamics. Particle trajectories are modeled from the nozzle chamber to the impact with the substrate. Optimum particle size is identified for various particle characteristics and spray configurations.