The Sea of Clouds is dotted by land masses called either bergs or cays, depending on the size. Bergs are larger than cays, and can be large enough to hold an entire nation, some are even as big as continents.
Cays are any land mass large enough to hold man made structures, but not large enough to be considered a berg. Some are only large enough for a single home while others house cities. There is no exact measure that determines whether any particular land mass is a berg or a cay, though it is generally accepted that any land mass that can hold multiple cities is clearly a berg.