Body size is a critical axis of biodiversity and is an important factor influencing extinction risk. Whether small- or large-bodied organisms are more extinction prone has received mixed support in various ecological contexts. Slow demographic rates that prevent quick recovery from disturbance and human exploitation can place large-bodied species at greater extinction risk, whereas limited dispersal abilities and range sizes can underly increased extinction risks for small-bodied species. Freshwater mussels (order Unionoida) are a highly threatened group of generally long-lived, slow-growing, filter-feeding bivalves that exhibit interspecific variation in body size that is linked to demographic rates and can influence species’ responses to human exploitation and habitat alterations, such as fragmentation by dams. Evaluating the relationship between body size and extinction risk could offer insight into the causes of mussel imperilment. Here, we used a species-level trait dataset for mussels of the USA to test if and how body-size and area of occupancy (AOO) were related to extinction risk. Because dispersal underlies the body size-extinction risk hypothesis and long-distance dispersal of mussels relies on the dispersal ability of a vertebrate hosts, typically fishes, during the parasitic larval life phase, we also addressed how host-fish dispersal distances and the number of dams occurring in a species’ AOO could relate to imperilment. Imperiled mussels were typically smaller and had narrower AOOs than non-imperiled mussels. Host-fish of non-imperiled mussel species had greater dispersal distances than those of imperiled mussel species, and the number of imperiled species increased where tall, large river dams (>12m dam height) were most frequent. As such, species with smaller bodies, which are often found in mussel diversity hotspots, and those with limited host dispersal are typically imperiled and thus could likely benefit from increased conservation attention, especially those species occupying rivers impounded by large dams.