Body size and reproductive rates are key characteristics of any organism. Size and reproduction are strongly influenced by environmental conditions, but individuals also influences their environment. The resulting feedback loop can generate ecological and evolutionary dynamics. In this thesis, I explore the causes and consequences of intraspecific variation in size and reproduction. Trinidadian guppies (Poecilia reticulata) are an ideal system for this purpose. Natural populations of guppies have evolved two distinct ecotypes that differ in terms of the sizes at which they are born and reach maturity, and in their reproductive rates. These ecotypes are adapted to different environments that vary in the intensity of predation risk, and in the availability of food. In this thesis, I use a combination of genetic, phenotypic, and environmental data from natural populations, experimental introductions, and controlled laboratory experiments. To better understand the causes and consequences of variation in body size and reproduction, I apply and test methods from three distinct approaches: quantitative genetics, dynamic energy budget theory, and life history theory. I quantify evolutionary change in a wild populations, and show that phenotypic change is masked by environmental degradation. I demonstrate why predictions of evolution can fail when environments change over time. I show how size and reproduction are linked through energetic assimilation, and demonstrate that smaller guppies are resource-limited in the wild. Using dynamic energy budget theory, I identify consistent differences in energetic allocation strategies between guppy ecotypes. Finally, I construct evolutionarily-explicit population simulations, where ecological and evolutionary dynamics are emergent properties of individual rates of growth, survival, and reproduction. I show that size-dependent competitive ability determines the outcome of life-history evolution in these simulations. My thesis presents solutions to problems relevant to many studies of the eco-evolutionary dynamics of wild populations.