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A method for the determination of average cluster size in supersonic beams is presented. Based on angular distribution broadening of the beams caused by passing through a buffer gas, this method is well suited for in situ determination of the mean cluster size when the apparatus contains a movable detector with sufficient spatial resolution. The shape and width of the beam profile after scattering by a buffer gas are evaluated theoretically as functions of buffer gas pressure and atom-cluster collision cross-section. Experimental results are presented for an argon beam, yielding average cluster sizes between 300 and 7000 atoms depending on the stagnation pressure. Simple criteria to assess the applicability of the method to a given experimental situation are discussed. The average cluster sizes determined in this work agree quite satisfactorily with previously published values for similar beam generation conditions. [ABSTRACT FROM AUTHOR]