The question of size is of overwhelming importance for small particles. To define the question we need to state precisely what we mean by the word size. It may mean some geometric size, which is relevant for obvious reasons, and which has given the name to the field of nanoscience (a branch of science where the studied objects are less than a micrometer for at least one dimension); it may mean the number of monomers in a particle, and it may mean the effective number of degrees of freedom. We will consider the latter aspect in this chapter, and we will disregard a number of effects that are much more important in other connections, such as quantum size effects where properties vary with the addition of a single monomer or a single electron to the particle.