In a crossed molecular beam arrangement helium atoms are scattered from argon clusters which are produced in an averaged size range of (n) over bar = 6 to (n) over bar = 90 by adiabatic expansion through sonic and conical nozzles. The diffraction oscillations in the total differential cross section are used to derive information on the size distribution of the clusters by comparison with quantum mechanical calculations based on a model potential. In the size range covered by the measurements, the average cluster size is given by (n) over bar = 38.4(Gamma*/1000)(1.64), where Gamma* is the scaling parameter of the source conditions introduced by Hagena [Z. Phys. D 4, 291 (1987)]. The results are in agreement with recent measurements of corrected mass spectra but disagree with the results obtained from electron diffraction. General relations are recommended which connect the scaling parameter with the averaged size.