Many plant and animal aggregations have size hierarchies within which a variety of sizes of individuals, from large to small, can be found. Size hierarchies are thought to indicate the existence of competition amongst individuals within the aggregation, but determining their exact cause is difficult. The key to understanding size hierarchies lies in first quantifying the pattern of size and growth of individuals. We conducted a quantitative investigation of pattern in the size hierarchy of the clown anemonefish Amphiprion percula, in Madang Lagoon, Papua New Guinea. Here, groups of A. percula occupy sea anemones (Heteractis magnifica) that provide protection from predators. Within each anemone there is a single group composed of a breeding pair and zero to four non-breeders. Within each group there is a single size hierarchy; the female is largest (rank 1), the male is second largest (rank 2), and the non-breeders get progressively smaller (ranks 3-6). We demonstrate that individuals adjacent in rank are separated by body size ratios whose distribution is significantly different from the distribution expected under a null model-the growth of individuals is regulated such that each dominant ends up being about 1.26 times the size of its immediate subordinate. We show that it is decisions about growth at the individual level that generate the size hierarchy at the group level, and thereby determine maximum group size and population size. This study provides a new perspective on the pattern, causes and consequences of size hierarchies.