3-D discrete dislocation dynamics simulations were used to investigate the size-dependent plasticity in polycrystalline, free-standing, thin films. A simple line-tension model was used to model the dislocation transmission cross grain boundaries. At a constant film thickness, the total dislocation density and the strength increase as grain size decreases. The yield stress scales with grain diameter with a power law, with an exponent that varies with both film thickness and grain size for thicker films. In addition, the yield strength of films scales proportionally to the reciprocal of thickness and matches experiment results well. A spiral source model was developed that relates the strength of films to the statistical variation of the spiral source length, and accurately predicts the size-dependent strength in polycrystalline thin films.