In this paper, we present a method to predict average droplet diameter distribution in a spray obtained by gas-assisted premixed atomization, and compare the results with experimental findings. The method is based on a two-fluid Eulerian-Eulerian treatment of the motion of the phases with a catastrophic phase inversion (atomization). It also includes the compressibility effects for the gaseous phase and can be applied to both the flow through the nozzle atomizer and to the dispersion of the spray. The model accounts for the breakup and coalescence of bubbles and droplets due to interfacial shear and collisions. The diameter of the particle (bubble or droplet) is represented by its local mean average value, which varies throughout the flow field. Simulations are conducted for the flow of air and water through the convergent-divergent nozzle, which is similar to the one used in commercial fluid cokers, a bitumen upgrading reactor, for steam-assisted atomization of bitumen. It is found that while there are wide experimentally observed local distributions of the particle diameter, the concept of the average diameter still allows for satisfactory predictions of its average values and spatial variations. We also investigate the influences of liquid surface tension and viscosity on spray properties.