South Dakota (USA) is 638 times larger than Saint Helena.
Area: 308 km2 World Factbook
Population: 7,776 (July 2014) World Factbook
Saint Helena, Ascension and Tristan da Cunha is a British Overseas Territory in the southern Atlantic Ocean consisting of the island of Saint Helena, Ascension Island and the island group called Tristan da Cunha. Wikipedia
South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux Native American tribes. South Dakota is the 17th most extensive, but the 5th least populous and the 5th least densely populated of the 50 United States. Wikipedia