Here's a breakdown of the geologic time scale from largest to smallest:

* Eon: The largest division of geologic time.

* Era: A subdivision of an eon.

* Period: A subdivision of an era.

* Epoch: A subdivision of a period.

While epochs are the smallest formally recognized unit, it's worth noting that chronostratigraphic units can be further subdivided into smaller units like ages and zones. However, these are not part of the standard geologic time scale.