According to Plato it was an island but also a civilization that expanded in both directions (east and west of it).

Kritias:

Our writings say how your state (means Athens) once won up on a powerful force, that starting with arrogance from a far away point of the Atlantic ocean surged against the whole of Europe and Asia. Indeed, back then, one could cross the sea because of its entrance, the one you call Pillars of Hercules (strait of Gibraltar), there was an island larger than Libya and Asia together.
From this island back then one could go to other islands and then across the continent, which was around the real ocean.

In this island, Atlantis, there was a great organization of kings with huge and formidable force, dominating all that place, many other islands and parts of the continent. In addition, the kings ruled several countries inside the strait (he means Gibraltar) like Libya and Egypt, and Europe up to Tyrrhenian (the sea, western coast of Italy and the islands there).