The mechanisms that control the size of cells, organs, and organisms have long interested biologists, and are also important in malignant progression. Despite this, basic features of in vivo growth control - the level at which regulation is exerted and the degree to which size is controlled autonomously - are poorly understood. Similarly, it is unknown whether adult tissues measure and respond to size cues in the same manner as those of embryos, a question that is relevant to tissue homeostasis and regeneration. This article will review the determinants of organ size during development and discuss the concept of a 'size set-point,' which incorporates the mass of tissue parenchyma and scaffold established at the end of development.