Tropical cyclone is one of the most important natural disasters on Earth. The structure and size of tropical cyclones not only impact their flooding and damaging area directly, but also influence their intensity and track indirectly. As an important basic scientific question, the scientific understanding of structure and size of tropical cyclones would improve the forecast skills of tropical cyclone’s intensity and track. Tropical cyclone size generally refers to the radius of maximum wind and the outer radius, while its structure is generally featured by the ratio between the two sizes. The size and structure of tropical cyclones in nature can differ significantly individually and change greatly during the evolution of tropical cyclones and thus impact their intensity and track. This paper briefly reviews the current research about structure and size of tropical cyclones from three perspectives (observations, modeling and theory). Theoretical derivation of Emanuel-Rotunno tropical cyclone model is also introduced. It also outlined the future research directions with potential ideas and thoughts to move forward.