After that, definitions are debated, but people generally agree that anything more than 230 feet is a megayacht, and more than 295 is a gigayacht. The world contains about 5,400 superyachts, and about a hundred gigayachts. For the moment, a gigayacht is the most expensive item that our species has figured out how to own. […] Many of the largest, most flamboyant gigayachts are designed in Monaco, at a sleek waterfront studio occupied by the naval architect Espen Øino.
Source: wiktionary