A simplified model for particle formation in emulsion polymerization (comprising aqueous-phase propagation to degrees of polymerization which may enter a pre-existing particle and/or form new particles by homogeneous or micellar nucleation, coupled with the aqueous-phase and intra-particle kinetics of oligomeric radicals) is formulated to provide a model suitable for the simulation of systems containing large-sized particles. The model is particularly useful to explore conditions for growth of large particles while avoiding secondary particle formation. Applied to the Interval II emulsion polymerization of styrene with persulfate initiator at 50°C, it is found that there is an effective maximum particle size that can be achieved if the formation of new particles is to be avoided. The parameter space of initiator concentration, particle number concentration and particle radius is mapped to show a catastrophe surface at the onset of new nucleation. Advanced visualization techniques are used to interpret the large number of simulations in the series, showing a maximum achievable particle diameter of around 5 um.