51 squares is about two-fifths as big as Fort Knox.
In other words, it's 0.4020 times the size of Fort Knox, and the size of Fort Knox is 2.490 times that amount.
(a.k.a. the United States Bullion Depository, a.k.a. "Gold Vault") (Kentucky) The United States Bullion Depository, commonly referred to by the metonym "Fort Knox" (in spite of the fact that the Depository is not located on the Fort proper), measures 32.0 m (105 ft) by 36.9 m (121 ft), for a total area of about 127 squares. The Depository is most famous for housing the largest portion of the national gold reserve, but has also temporarily housed the original US Declaration of Independence, an original copy of the US Constitution, and other important documents, kept there for safe keeping during World War II.