Fort Jefferson National Monument was designated on January 4, 1935. Congress redesignated it Dry Tortugas National Park on October 26, 1992.
The history page says the monument comprised 47,125 acres, was expanded in 1983, and became a park to protect island and marine ecosystems, Fort Jefferson, submerged cultural resources such as shipwrecks, and regulated public access. The keys sit about 70 miles west of Key West.
Seven keys make the Dry Tortugas: Garden, Loggerhead, Bush, Long, East, Hospital, and Middle. The park homepage calls the whole place about 100 square miles, mostly open water. Fort Jefferson, on Garden Key, is described there as the largest masonry structure in the Americas, more than 16 million bricks.