Body size is a key functional trait of species and large-sized marine fishes suffer most from exploitation. Using maximum length data from >12 000 marine fish species within 56 large marine ecosystems (LMEs), Fisher and colleagues document strong latitudinal and longitudinal variations in mean length and shape of size distributions. Size peaked in North Atlantic LMEs where distributions were skewed towards large species and where declines in ecosystem functioning (measured by fisheries landings) have been most severe. These patterns and a strong negative correlation between mean size and species richness suggest that differences in species size structure underlie both the differential resilience of LMEs to size-selective exploitation and positive relationships between species richness and global LME functioning.