The area of Fort Lytton National Park is 130,000.0 square meters.
The area of Fort Clinch State Park is 4,451,542.06464 square meters.
The area of Fort Glanville Conservation Park is 50,000.0 square meters.
The area of Fort Harrison State Park is 6,879,655.91808 square meters.
The area of Melrose Park - Fort Lauderdale - is 2,300,000.0 square meters.
The area of Fort Ord Dunes State Park is 3,965,919.2939520003 square meters.
The area of Fort Pierce Inlet State Park is 1,375,931.183616 square meters.
The area of Fort Ross State Historic Park is 13.731 square kilometers.
The area of Fort Humboldt State Historic Park is 72,843.4156032 square meters.
The area of Fort Pierre National Grassland is 468.99 square kilometers.
The area of Fort Matanzas National Monument is 921,712.018765824 square meters.
The area of Fort Frederica National Monument is 1,151,290.1836085762 square meters.
First of all, Fort Custer Recreation Area is a state park, not a national one. It is "man-made" in the sense that most of the recreational improvements were built by humans. The lakes, however, were present when the land was taken for military training. Interestingly, no fort was ever built on the land.