  Plastic deformation of heterogeneous solid structures is often characterized
by random intermittent local plastic events. On the mesoscale this feature can
be represented by a spatially fluctuating local yield threshold. Here we study
the validity of such an approach and the ideal choice for the size of the
representative volume element for crystal plasticity in terms of a discrete
dislocation model. We find that the number of links representing possible
sources of plastic activity exhibits anomalous (super-extensive) scaling which
tends to extensive scaling (often assumed in weakest-link models) if quenched
short-range interactions are introduced. The reason is that the interplay
between long-range dislocation interactions and short-range quenched disorder
destroys scale-free dynamical correlations leading to event localization with a
characteristic length-scale. Several methods are presented to determine the
dynamic length-scale that can be generalized to other types of heterogeneous
materials.
