This paper presents a summary of several seminal observational and numerical studies on climatology and possible change mechanisms of the outer-core wind structure of a tropical cyclone (TC). This has generally been referred to as size, a term also used in this review despite various definitions being given in the literature. In all the ocean basins where TCs exist, TC size has been found to vary with season, year, decade, latitude and longitude. Such variations are related to the synoptic flow patterns in which the TCs are embedded. Several factors have been identified as responsible for changes in TC size, including: environmental humidity, vortex structure, sea surface temperature and planetary vorticity. Each of these factors can modify the transport of lower tropospheric angular momentum into the TC and cause changes in its size. The paper ends with a discussion of outstanding issues in the study of the outer-core wind structure of a TC.