The sizes and shapes of marine organisms often vary systematically across latitude and water depth, but the environmental factors that mediate these gradients in morphology remain incompletely understood. A key challenge is isolating the individual contributions of many, often correlated, environmental variables of potential biological significance. Benthic foraminifera, a diverse group of rhizarian protists that inhabit nearly all marine environments, provide an unparalleled opportunity to test statistically among the various potential controls on size and volume–to–surface area ratio. Here, we use 7035 occurrences of 541 species of Rotallid foraminifera across 946 localities spanning more than 60 degrees of latitude and 1600 m of water depth around the North American continental margin to assess the relative influences of temperature, oxygen availability, carbonate saturation, and particulate organic carbon flux on their test volume and volume–to–surface area ratio. For the North American data set as a whole, the best model includes temperature and dissolved oxygen concentration as predictors. This model also applies to data from the Pacific continental margin in isolation, but only temperature is included in the best model for the Atlantic. Because these findings are consistent with predictions from the first principles of cell physiology, we interpret these statistical associations as the expressions of physiological selective pressures on test size and shape from the physical environment. Regarding existing records of temporal variation in foraminiferal test size across geological time in light of these findings suggests that the importance of temperature variation on the evolution of test volume and volume–to–surface area ratio may be underappreciated. In particular, warming may have played as important a role as reduced oxygen availability in causing test size reduction during past episodes of environmental crisis and is expected to inflict metabolic stress on benthic foraminifera over the next century due to anthropogenic climate change.