The body size distributions of animal populations are widely assumed to reflect their status and resilience. While body size distributions are shaped by complex interactions among growth, mortality, and recruitment, ecological constraints are likely to result in generalisable patterns. We used survey data for 3,228 populations and 822 species of shallow water marine fishes to investigate how well body length distributions within populations and species could be predicted without knowledge of underlying ecological processes. Scaling all observed fish lengths to the mean body length of the population revealed generally consistent unimodal size distributions with similar variation around the mean. This observation was evident across two orders of magnitude in fish maximum body lengths and diverse life-histories. Moreover, scaled intra-specific body size variability in natural fish populations was similar to that observed in protists and rotifers. The observed predictability in species-level size distributions suggests a narrow range of outcomes from multiple intrinsic and extrinsic factors influencing body sizes. Our findings have critical implications for size-based assessments of fish stocks worldwide, providing new opportunities for management and an improved understanding of human impacts on wild fish populations.