According to the San Francisco Recreation & Parks Department, the size of Civic Center Plaza ranges from 4.53 to 5.38 acres. It was originally commissioned after San Francisco for the 1915 Panama–Pacific International Exposition. The plaza was reconstructed starting in 1956 to build two underground structures: Brooks Hall and a parking garage and includes two play structures which opened in 1999.
This area has been at the heart of San Francisco activity since it was created.
Tons of images can be found at the Open SF History website.