Assembly rules based on body size characterize processes that determine community composition and structure. One prominent model proposes a spatial scaling law (SSL) that links body size with foraging behavior and predicts the minimum difference in body size that is necessary for species coexistence. Although this SSL is cited frequently, robust tests of its predictions are few, and its performance in these tests has been mixed. We used data on 34 well sampled bat assemblages from throughout the New World to test predictions of the SSL for 5 feeding guilds: aerial insectivores, frugivores, high-flying insectivores, gleaning animalivores, and nectarivores. Contrary to the model's predictions, body-size ratios of species of adjacent size did not decrease with increasing body size, the frequency distribution of sizes within a guild was not left-skewed, and the relationship between species richness and productivity was not modal with a long tail to the right. Body size alone appears insufficient to describe niche differentiation and species coexistence in New World bats, calling into question the broad applicability of this model of spatial scaling. Future studies of the SSL should identify the characteristics that predispose a community to be characterized well by such a model, rather than assuming it is a robust descriptor of communities regardless of taxon and other conditions.