By introducing periodic flow inversions, we have shown both experimentally and computationally that forcing with a value above a critical frequency can effectively eliminate both density and size segregation. The critical frequency is the key to applying this technique and is directly related to the inverse of the characteristic time of segregation. In this work, we study size segregation and propose a generalized size segregation model to determine the critical forcing frequency in these systems. The influences on mixing of the length of each leg of the zig-zag chute and the size ratio of the particles are examined. Mixing is observed instead of segregation when L<U_avgt_S, where L, U_avg, and t_S denote the length of each leg of the zig-zag chute, the average stream wise flow velocity of the particle, and the characteristic time of segregation, respectively.