Hybrid quantum-classical molecular-dynamics simulations were performed for the dissociation process of HNO3 in aqueous clusters containing up to n = 124 water molecules. By circumscribing the HNO3 and one water molecule within the quantum region, no dissociation was observed, even for the largest cluster considered, in which the process is expected to be spontaneous. Incorporation of two additional water molecules into the quantum sub-system was necessary to observe the proton transfer. Ionic dissociation takes place for n=124 and n=10, but not for the n=3 case, confirming that the threshold cluster size for dissociation is intermediate between the last two values.