nLab
small site

Context

Topos Theory

topos theory

Background

Toposes

Internal Logic

Topos morphisms

Extra stuff, structure, properties

Cohomology and homotopy

In higher category theory

Theorems

By a “small site” is typically meant a site whose underlying category is a small category. This is the default case, but may be used, for emphasis, in contrast to large site.

But beware that there is also the, unrelated, concept of a little site.