  Plastic deformation of crystals proceeds through a sequence of intermittent
slip avalanches with scale-free (power-law) size distribution. On macroscopic
scales, however, plastic flow is known to be smooth and homogeneous. In the
present letter we use a recently proposed continuum model of slip avalanches to
systematically investigate the nature of the cut-off which truncates scale-free
behavior at large avalanche sizes. The dependence of the cut-off on system
size, geometry, and driving mode, but also on intrinsic parameters such as the
strain hardening rate is established. Implications for the observability of
avalanche behavior in microscopic and macroscopic samples are discussed.
