A theoretical model to predict the joint distribution of droplet size and charge density for an electrostatic spray is described based on the maximum entropy method. From known values of the electrostatic spray parameters, the model is used to evaluate the joint distribution of droplet size and charge density for a cone-jet mode electrostatic spray. The predicted results of present model show that it has generally a relatively narrow distribution for both droplet and charge density in a cone-jet mode. Comparatively, the droplet size distribution is narrower than that of the charge density. In addition the two distributions are significantly different in shape. The droplet size distribution is nearly symmetric about its peak position, whilst the left side of the charge density distribution curve is noticeably steeper than the right side. The results are also compared with existing experimental data with agreement considering the uncertainties in the data.