Why do things have the size that they do? Where does the size of something come from? We can tell it's a deep question because it sounds as one a child would ask. Mechanics offer an answer. A given size or length can be understood as the result of a balance or ratio between opposing mechanical factors, one tending to grow and the other to shrink. In this second video of this series on mechanics we give a tour of the many types of sizes that can be understood from ratios of pairs of mechanical quantities. These simple lengths are found everywhere, from atoms to astronomical objects like planets, stars and supernova remnants.