Named after the protagonist from the children's story The Three Bears. Conceptually, "just the right amount":

In astrobiology, the Goldilocks zone refers to the habitable zone around a star, roughly corresponding to the temperature gradient survivable by life.

There are tons of other examples. Too much of a good thing can often be bad, as described in Ilich's "Two Watersheds" parable (see Two Watersheds - too much institutionalization), and this applies in a variety of areas (see Too much of a virtue can become a vice, and Perils of optimization).

This is also related to the Buddhist concept of the middle way (see Middle way in Buddhism), and Lagom, a Swedish word that has no direct translation. It more or less means not too much, not too little, just the right amount.

In technological endeavors, it may be difficult to build a project if you have too few people, but too many people can also be hazardous (see Scale causes ossification and Reaching maximum scale is sometimes a bad idea).