Bulk polystyrene (PS) undergoes craze-assisted brittle failure at room temperature. In this work it is shown that the synergistic coupling of the material length scale as defined by macromolecular size, and the specimen size as defined by the nanofiber diameter, can result in extreme ductility and simultaneous strengthening and toughening for fiber diameters at the submicron scale. Combinations of PS fiber diameters between 150 nm and 5000 nm, and molecular weights between 13,000 g/mol and 9,000,000 g/ mol provided a spectrum of degrees of molecular confinement, which, for particular pairs of molecular weight and fiber diameter, allowed for large fiber extensions through the process of necking followed by large homogenous deformation controlled by strain hardening. Compared to bulk PS, the combined necking and post-neck strain hardening increased the fiber strength by 350% and the fiber ductility by as much as 4000%. This mechanical response was shown to scale well with the confining parameter D-norm, defined as the ratio of the initial fiber diameter (D-0) to the root-mean-square end-to-end distance (R-ee), with D-norm similar to 18 identified as the threshold for the transition from necking to crazing of PS nanofibers. The large deformation response of PS nanofibers with molecular weights above a threshold value is shown to obey a master true stress vs. normalized strain curve that is independent of molecular weight. (C) 2014 Elsevier Ltd. All rights reserved.