Body size is one of the basic traits of animals and is regulated to adapt to the environment. Animals perceive environmental stimuli with sensory neurons, and signals from the nervous system alter the size of organs, thus regulating body size. The model animal Caenorhabditis elegans is particularly suited for genetic analysis of body size regulation, and has already contributed to the elucidation of various genetic pathways that regulate body size. In this review, we summarize the available literature regarding environmental factors that regulate body size and the role of the nervous system in such regulation. We discuss in detail a recent report on body size regulation by the neurotransmitter, dopamine.