Theoretical models of lineshapes in Ar2p photoionization spectra have been calculated for free, neutral argon clusters of different sizes. The lineshape models are fitted to experimental spectra and used to estimate the mean cluster size realized in the experiment. The results indicate that size estimators working from stagnation conditions [R. Karnbach, M. Joppien, J. Stapelfeldt, J. Wörmer and T.Möller, Rev. Sci. Instrum., 1993, 64, 2838] may underestimate the mean cluster size.