A model is proposed that describes the development of individual and multiple necks in superplastically deformed materials. Within the model, the examined samples have the form of round bars and are subjected to tensile superplastic deformation without strain hardening.  It is demonstrated that neck development and necking-induced failure occur faster with a decrease in strain rate sensitivity and/or an increase in the fraction of the sample length occupied by necks. This means that high values of strain to failure observed in small specimens of superplastically deformed ultrafine-grained metals and alloys, where diffuse necking happens in the whole specimen, can be significantly reduced in larger specimens where the necking regions occupy only a small part of the sample.