When molecules approach each other at distances typical of gas-phase complexes or condensed-phase media, it is known that the intermolecular electron density function in the region between the molecules is related to the strength of the intermolecular interaction. We explore this behavior for 50 interaction pairs, and find that, in the interaction region, the total electron density is well represented by the sum of the density functions of the isolated molecules. The minimum in the electron density function between pairs of interacting molecules is used to estimate the sizes of the molecules. Taken in conjunction with the density additivity in this region, this procedure provides a means of estimating molecular sizes without performing supermolecule calculations. For weakly interacting systems, the distances and density minima identified by this procedure are consistent with use of the 0.002 an isodensity surface to define the size and shape of a molecule in condensed media.