Rubber toughened epoxies are used in a wide range of applications including adhesives when toughness is a crucial property. It is well known that the cavitation of the rubber particles is an important process to optimise the toughness of such materials. This article describes the development of a predictive model to describe the dependence of rubber particle cavitation on particle size. The model is developed using a combination of experimental observations and finite element simulations. Predictions have been obtained for both uniaxial loading conditions and the triaxial loading conditions expected ahead of a crack. The model has been extended to consider the cavitation of nano-sized 'rubber' particles.