Non-Equilibrium Molecular Dynamics (NEMD) simulations have exhibited characteristic velocity weakening for the tangential frictional force at smooth single crystal interfaces for velocities greater than a critical velocity, vc. This behavior has been seen in a number of material pairs including Cu-Ag, Ta-Al and Al-Al. Expressions for vc that characterize this behavior depend on system size. We discuss the size dependence for Al-Al single crystal interfaces for two cases, an Al(111)/Al(001) interface sliding along [1-10], N=1.5 106, and an Al(110)/Al(110)[1-10] interface sliding along , N=7.5 106, where N is the number of atoms, corresponding to a three-fold increase in system size normal to the sliding direction. We find agreement with an inverse size scaling for vc.