We present a unified mechanistic model to rationalize size-dependent flow stress, activation volume and strain-rate sensitivity for metals with either nanocrystalline grains or nanoscale twins. The non-uniform partial dislocation model for flow stress [Asaro and Suresh, Acta Mater, Vol. 53, pp. 3369–3382, 2005; Gu et al., Scripta Mater, Vol. 62, pp. 361–364, 2010] is generalized here to consider both grain-size dependence and twin-thickness dependence of nanotwinned metals. A non-homogeneous nucleation model is proposed to predict the dependence of activation volume on both grain-size and twin-thickness. With the activation volume predicted from the non-homogeneous nucleation model and the flow stress obtained via the non-uniform partial dislocation model, strain-rate sensitivity as a function of characteristic structural length scale is also evaluated. This provides a unified approach from envisioning partial dislocation emission for the three size-dependent parameters characterizing the plastic deformation mechanism, flow stress, activation volume and strain-rate sensitivity, so that each one of these parameters is predicted when the geometry of the grains or nanotwins is known. The model predictions are shown to be consistent with a variety of available experimental data.