This paper introduces a Breakage Mechanics model formulated in the framework of Cosserat continuum theory. This formulation allows us to account for the interaction of grain size evolution with shear band formation in a physically motivated and thermodynamically consistent way. We provide an upscaling procedure that introduces material parameters accounting for the contribution of the Cosserat rotations, as well as the micro-inertia of the entire grain size distribution. A particular highlight of the new formulation is the capacity to describe an evolving internal Cosserat length that takes account of the complete grain size distribution and its evolution. In addition, we specify a particular model that requires no additional calibration relative to the same model in the classical continuum. We apply this model to the study of a layer sheared under constant volume which simulates the fast undrained deformation observed in seismogenic faults. Through linear stability analysis we use the model to examine the effect of grain size polydispersity on the thickness of shear bands. By implementing the model using the finite element method we provide an explanation for the geological formation of double cataclastic shear bands.