In a dispersed multiphase flow, once the particle fluid interaction in DPM, DDPM and Eulerian Models are defined as empirical models for sub-grid particles (e.g.: Schiller Naumann Drag Model), the particle diameter must be much smaller than cell size to account the interaction between phases accurately. If the particle diameter are larger than cell size, you must adopt the Macroscopic Particle Model, in which the interactions between phases are determined as part of the solution. In this case, particles span many fluid cells.