The unambigious definition would be a contiguous landmass surrounded by water at sealevel, which would be in descending order of area

Afro-Eurasia *

Americas *

Antarctica

Australia landmass

Greenland

* This of course depending on how rivers and man-made canals of Suez, Panama, and other canals like Kiel, or ones linking rivers, are treated, there could be arguments that Suez should be included (no locks), but Panama shouldn't be, or all should be, or none should be, or Suez and Panama should be, etc