Body size in C. puella has been shown to have opposing costs and benefits in terms of the animals’ lifetime mating success. Large size is positively correlated with increased longevity in males and females, but negatively correlated with daily mating rate in males and clutch size in females. The optimum sizes for males and females were calculated in terms of their expected lifetime mating successes. The predicted optimum male size fell close to the mean male size observed in the field. The optimum female size was larger than that observed for females in the field. There is evidence for stabilizing selection in male damselflies and weak directional selection in females. The predicted optimum female size is larger than that for males; this is a sufficient explanation for the sexual size dimorphism.