Small categories are free of some of the subtleties that apply to large categories.

A category is said to be essentially small if it is equivalent to a small category. Assuming the axiom of choice, this is the same as saying that it has a small skeleton, or equivalently that it is locally small and has a small number of isomorphism classes of objects.

A small category structure on a locally small category$C$ is an essentially surjective functor from a set (as a discrete category) to $C$. A category is essentially small iff it is locally small and has a small category structure; unlike the previous paragraph, this result does not require the axiom of choice.

Smallness in the context of universes

If Grothendieck universes are being used, then for $U$ a fixed Grothendieck universe, a category $C$ is $U$-small if its collection of objects and collection of morphisms are both elements of $U$. $C$ is essentially $U$-small if there is a bijection from its set of morphisms to an element of $U$ (the same for the set of objects follows); this condition is non-evil.

So let $U\Set$ be the category of $U$-small sets. Then

a $U$-small category is a category internal to$U Set$.

A category is $U$-moderate if its set of objects and set of morphisms are both subsets of $U$. However, some categories (such as the category of $U$-moderate categories!) are larger yet.