For clusters produced by condensation from a gas phase, a basis for identifying the dimensionless thermodynamic and kinetic parameters which characterize cluster formation is provided by arranging an expression for the mass rate of production of critical-size clusters into dimensionless form. The experimentally observed dependence of the cluster size on source pressure dictates the choice between two alternative classical expressions for the nucleation rate. An earlier analysis of dimer formation in free jets guides the combining of the thermodynamic and kinetic parameters into a single scaling parameter which facilitates correlating the size of clusters in free jets for all rare gases, including He-4, using a single curve. Important facets of the correlation are (a) the explicit dependence of the scaling parameter on the surface tension and, for the rare gases other than He-4, the evaluation of the surface tension at the triple point, and (b) the use of real-fluid properties for the density and sound speed of the He-4 in the source. An earlier conclusion that size distributions for large condensation clusters can be correlated with dimer partial pressure in the source is found to be fortuitous. (C) 1997 American Institute of Physics.