Body size is important in determining the outcome of competition and predator-prey interactions. Size structure of a population (i.e. relative proportion of large and small conspecifics) may be particularly important in organisms with prolonged breeding periods and rapid growth where populations may have multiple cohorts at different stages of development competing for one resource. Both the consumptive and nonconsumptive effects of predators can also be size-dependent and can alter competitive interactions. Here we study the importance of size structure in the Neotropical leaf-breeding tree frog, Agalychnis callidryas. This species is a prolonged breeder such that multiple overlapping cohorts of differing sizes are common. Specifically, we examine size-specific intraspecific competition between A. callidryas tadpoles and then explore how predation affects these interactions. To determine the strength of inter-cohort competition, we manipulated the density and relative proportion of large and hatchling tadpoles in a response surface design and quantified growth. We then observed the effect of a dragonfly larvae predator (Anax amazili) on tadpole growth and survival at different size-structured treatments. Large tadpoles were greater per individual competitors while hatchlings were greater per gram competitors. When predators were added, dragonflies reduced survival and growth of hatchlings substantially, but had no effect on large tadpoles. Further, dragonflies reduced hatchling growth more when other hatchlings were present. The predator effect on hatchling growth was 23% larger than the effect of competition with large tadpoles, such that the importance of size structure for A. callidryas may be mediated more through predation than intercohort competition.