Given a size pair (M,f)\ where M\ is a manifold of dimension n\ and f\ is an arbitrary real continuous function defined on it, the i-th size functor, with, denoted by F_i, is the functor in , where is the category of ordered real numbers, and is the category of Abelian groups, defined in the following way. For x\le y, setting, , j_{xy}\ equal to the inclusion from M_x\ into M_y, and k_{xy}\ equal to the morphism in from x\ to y, In other words, the size functor studies the process of the birth and death of homology classes as the lower level set changes. When M\ is smooth and compact and f\ is a Morse function, the functor F_0\ can be described by oriented trees, called H_0\ − trees. The concept of size functor was introduced as an extension to homology theory and category theory of the idea of size function. The main motivation for introducing the size functor originated by the observation that the size function can be seen as the rank of the image of. The concept of size functor is strictly related to the concept of persistent homology group, studied in persistent homology. It is worth to point out that the i-th persistent homology group coincides with the image of the homomorphism.