Is Manhattan Island the largest island

Not hardly.

-- The area of Manhattan Island is 22.82 square miles.

-- Almost right next door to Manhattan Island lies Long Island, also in New York State.  A big part of Long Island is the Brooklyn and Queens boroughs of New York City.  The area of Long Island is 1,401 square miles.  That's 61 times the size of Manhattan Island.

-- 18 states in the US have islands bigger than Manhattan Island.

-- 2 states in the US have islands bigger than Long Island.

-- Hawaii is an Island.  It's 2.9 times the size of Long Island.

-- There are 9 islands in the US that are bigger than Long Island.  Hawaii is one of them.  The other 8 are in Alaska.

-- The 100 largest islands in the world are all bigger than Long Island.  A few of them are:  Greenland,  Australia,  Antarctica,  Madagascar,  Honshu Island Japan,  Hokkaido Island Japan,  Kyushu Island Japan,  Great Britain,  Ireland,  New Zealand North Island,  New Zealand South Island,  Cuba,  Iceland,  Sri Lanka,  Tasmania,  Taiwan,  Sicily,  Sardinia,  Jamaica,  Cyprus,  Puerto Rico,  Corsica,  Crete.    

-- Antarctica itself is an island.  It also has smaller islands around its coast. ONE little island off the coast of Antarctica, called Alexander Island, is about 12 times the size of Long Island.