Is Cuba the largest island in the world?

No.

At 2,175,590 km2 in area (840,000 square miles), Greenland is the world's largest island.

Australia is often erroneously thought of as the world's largest island.

Geographically, Australia is a continent, not an island. As a landform, it could be considered an island as it is entirely surrounded by water and not joined onto any other land mass. For this reason, it is often referred to as an island continent. Australia is too big to be formally classified as an island. The world's largest island is Greenland.

Another reason why Australia is also considered a continent is because it sits on its own tectonic plate. This is different from Greenland, which uses the same land mass as North America.