Size distributions through large van der Waals cluster beam cross-section are studied with the pick-up technique. Based on our experimental results, we observed that the larger cluster is always concentrated in the center of the beam. From the center to the periphery, the cluster size gradually decreases. The size distributions through the beam cross-section depend on incoming cluster size and incoming cluster velocity. The larger the incoming cluster size or the faster the incoming cluster velocity, the flatter the size distributions through the beam cross-section are found. These experimental results are interpreted by the Mack focusing effect. (C) 2003 Elsevier B.V. All rights reserved.