Body size affects important fitness variables such as mate selection, predation and tolerance to heat, cold and starvation. It is therefore subject to intense evolutionary selection. Recent genetic and physiological studies in insects are providing predictions as to which gene systems are likely to be targeted in selecting for changes in body size. These studies highlight genes and pathways that also control size in mammals: insects use insulin-like growth factor (IGF) and Target of rapamycin (TOR) kinase signalling to coordinate nutrition with cell growth, and steroid and neuropeptide hormones to terminate feeding after a genetically encoded target weight is achieved. However, we still understand little about how size is actually sensed, or how organ-intrinsic size controls interface with whole-body physiology.