This chapter explains how to describe the three-dimensional shape of polymers in an average sense, and how the average size of the object in space will depend on molecular weight. It also explores the equilibrium distribution of sizes. The remarkable properties of polymers derive from their size. Another would be to look at a large collection of structurally identical chains at a given instant in time –– an ensemble average. It is an interesting fact that bulk polyethylene has a positive coefficient of thermal expansion, whereas the individual chain dimensions have a negative coefficient. In other words, when a piece of polyethylene is heated, the volume increases while the individual chain dimensions shrink. Thermal expansion corresponds to a decrease in the density of the material, which reflects primarily an increase in the average distance between molecules; the radii of the individual atoms and the bond lengths also tend to increase, but to a much smaller extent.