Locomotor performance is crucial to survival in many species. Swimming performance in fish depends on fin shape and size, and swimming performance may change with fin damage. The aim of this study was to investigate the relationship between fin size and swimming performance in male Eastern mosquitofish Gambusia holbrooki either with undamaged fins, or with fins that have sustained damage as a consequence of aggressive encounters. We show that in fish with undamaged fins burst swimming speeds increase with an increasing caudal fin size, while sustained swimming speeds (Ucrit) decrease with increasing fin size. In fish with damaged fins, Ucrit increases with an increasing caudal fin area, demonstrating a measurable cost of fin damage. The relationship between fin size and Ucrit is not linear but is best described by a Gaussian curve, where Ucrit decreases as fin size either increases or decreases from a central optimal value. We suggest that fish with large fins benefit because they can withstand more fin damage resulting from intraspecific aggression before experiencing detrimental effects such as reduced Ucrit.