Using electron attachment time-of-flight mass spectrometry, we show how high-precision structural constants of van der Waals aggregates may be obtained for two kinds of homogeneous clusters, (SF6)(N) and (CO2)(N). Furthermore, we obtain size-specific structural information over a wide range of aggregate sizes. Mass spectrometric data are presented regarding the size needed to facilitate the transition from "cluster packing," dominated by nearest-neighbor interactions, to bulk-like packing. For both examples, it appears that the cluster-to-bulk packing transition may occur even for aggregates where the majority of the molecules resides at the surface. The critical size for the cluster-to-bulk transition may be related to the size at which molecules packed as bulk crystals can begin forming nearly spherical shapes. A discussion of the mechanism by which geometrical shell closings are visualized in electron attachment time-of-flight mass spectrometry is also presented. We postulate that these observations reflect the dynamics of electron localization in ordered crystallites with and without defects.