Rayleigh scattering has been used to determine the average size of clusters formed in pulsed free-jet expansions of Ar and N2O. The scaling behavior of the mean cluster size with backing pressure predicted by Hagena (Rev. Sci. Instrum. 1992, 63, 2374) and Buck and Krohne (J. Chenz. Phys. 1996, 105, 5408) has been shown to be applicable up to at least an average cluster size of 25 000. A similar scaling of the average cluster size with pressure was observed for both Ar and N2O. Competing photochemical and photophysical processes are shown to limit the use of short-wavelength radiation to observe Rayleigh scattering from the N2O clusters.