  We investigate by numerical simulation the system size dependence of the
shear delamination strength of thin elastic films. The films are connected to a
rigid substrate by a disordered interface containing a pre-existing crack. The
size dependence of the strength of this system is found to depend crucially on
the crack shape. For circular cracks, we observe a crossover between a
size-independent regime at large crack radii which is controlled by propagation
of the pre-existing crack, and a size-dependent regime at small radii which is
dominated by nucleation of new cracks in other locations. For cracks of finite
width that span the system transversally, we observe for all values of the
crack length a logarithmic system size dependence of the failure stress. The
results are interpreted in terms of extreme value statistics.
