One of the most pressing aims in ecology is discerning the interrelationships among the environment, species diversity, and ecosystem functions. A wealth of theoretical and experimental research has shown that more diverse communities support higher standing biomass and energy fluxes in ecosystems (biodiversity–ecosystem function; BEF); however, BEF relationships in natural communities show much more complex and idiosyncratic patterns. This discrepancy is especially apparent in multi-trophic communities, where the relative importance of bottom-up and top-down control leads to diverse relationships between diversity and ecosystem functions such as total community biomass and energy flux. Recent work on the BEF relationships in multi-trophic communities has highlighted that complementary approaches, particularly allometric theory (AT), may help explain variability in BEF relationships across ecosystems. Here, we develop a framework leveraging a widely observed macroecological rule from AT—that organism abundance (N) is negatively related to organism body size (M) within and across ecosystems—to explore the environmental and community drivers of community standing stock biomass in the continental-scale stream gradient of the National Ecological Observatory Network (NEON). Using two size-abundance relationships: 1) the individual size distribution (ISD), which describes the relative frequency of body sizes within a community and represents a proxy of food web energetic efficiency, and 2) the cross-community scaling relationship (CCSR), which describes covariation in community abundance and body size across communities, we integrate BEF and AT concepts to explain patterns in community biomass. Streams across NEON vary >20°C in mean temperature, orders of magnitude in basal resource availability (i.e., gross primary production and organic matter standing stock) and, accordingly, communities of macroinvertebrate and fish vary multiple orders of magnitude in community abundance (104 –106 individuals/m2), biomass (101–103 g/m2), mean body size (101–103 mg/individual), and >3-fold in species richness (30–115 species). We find weak bivariate relationships between diversity and community biomass, but stronger congruence between predictions of AT and community biomass patterns. We further incorporate determinants of ecosystem energetics such as basal resource availability and temperature, with BEF and AT concepts towards developing a richer understanding of macroecological drivers of community biomass and ecosystem function.