It is observed that the mean slip in large earthquakes is linearly proportional to fault length and does not correlate with fault width. This observation is interpreted in the light of the two possible classes of models for large earthquakes: W models, in which stress drop and slip are determined by fault width, and L models, in which these parameters are fundamentally determined by fault length. In the W model interpretation, stress drop systematically increases with L/W, the aspect ratio, and, as a consequence, seismic moment. The correlation of slip with length means that the rupture length is determined by the dynamic stress drop. This conflicts with the observation that the length of large earthquakes is often controlled by adjacent rupture zones of previous earthquakes or by tectonic obstacles. It also conflicts with the observations for small earthquakes that stress drop is nearly constant and does not correlate with source radius over a broad range. In the L model interpretation, the correlation between slip and length means that stress drop is constant, namely about 7.5, 12, and 60 bars for interplate strike-slip, thrust, and Japanese intraplate earthquakes, respectively. L models require that the fault be mechanically unconstrained at the base. W models predict that mean particle velocity increases with fault length, but rise time is constant. L models predict the opposite.