Question: El Retiro is a historic site in Lake Wales, Florida, that since 1970 has been part of a national historic landmark that consists of how many acres of garden?

Context paragraphs:
El Retiro (Lake Wales, Florida): El Retiro (also known as Encierro and today as Pinewood Estate) is a historic site in Lake Wales, Florida.  It is located on Mountain Lake, off State Road 17.  Since 1970, it has been part of Bok Tower Gardens.  On December 12, 1985, it was added to the U.S. National Register of Historic Places.

Salem Maritime National Historic Site: The Salem Maritime National Historic Site is a National Historic Site consisting of 12 historic structures, one replica tall-ship, and about 9 acres (36,000 m) of land along the waterfront of Salem Harbor in Salem, Massachusetts.  Salem Maritime is the first National Historic Site established in the United States (March 17, 1938).  It interprets the Triangle Trade during the colonial period, in cotton, rum, sugar and slaves; the actions of privateers during the American Revolution; and global maritime trade with the Far East, after independence.  The National Park Service manages both the National Historic Site and a Regional Visitor Center in downtown Salem.  The National Park Service (NPS) is an agency of the United States Department of the Interior.

Harry S. Truman Historic District: The Harry S. Truman Historic District is a National Historic Landmark District encompassing sites closely associated with US President Harry S. Truman in Independence, Missouri.  It includes the Truman Home at 219 North Delaware, Truman's home for much of his adult life and now a centerpiece of the Harry S. Truman National Historic Site, and the Truman Presidential Library.  The district was listed on the National Register of Historic Places in 1971, and was designated a National Historic Landmark in 1974.  When first listed, the district included only the two buildings and a corridor joining them.  It was substantially enlarged in 2011 to include more sites and the environment in which Truman operated while living in Independence.

Puʻukoholā Heiau National Historic Site: Puʻ ukoholā Heiau National Historic Site is a United States National Historic Site located on the northwestern coast of the island of Hawaiʻ i.  The site preserves the National Historic Landmark ruins of the last major Ancient Hawaiian temple, and other historic sites.

Guthrie Historic District (Guthrie, Oklahoma): The Guthrie Historic District (GHD) is a National Historic Landmark District encompassing the commercial core of Guthrie, Oklahoma, US.  According to its National Historic Landmark Nomination it is roughly bounded by Oklahoma Avenue on the north, Broad Street on the east, Harrison Avenue on the south, and the railroad tracks on the west; it also includes 301 W. Harrison Avenue.  The National Historic Landmarks Program on-line document describes the boundaries as "14th Street, College Avenue, Pine Street and Lincoln Avenue.  One building, the Logan County Courthouse, is at 301 E. Harrison Avenue, outside the main boundaries of the GHD," This article relies on the former source, which is more detailed.  According to the 1998 nomination, the proposed district covered 31 acres .  The nomination included 112 resources, classed as 69 contributing buildings, 38 non-contributing buildings, 1 non-contributing structure and 3 noncontributing objects.  It was declared a National Historic Landmark in 1999 for its historic significance as the first capital of the Oklahoma Territory and of Oklahoma.

Schoharie Crossing State Historic Site: Schoharie Crossing State Historic Site, also known as Erie Canal National Historic Landmark, is a historic district that includes the ruins of the Erie Canal aqueduct over Schoharie Creek, and a 3.5 mi long part of the Erie Canal, in the towns of Glen and Florida within Montgomery County, New York.  It was the first part of the old canal to be designated a National Historic Landmark, prior to the designation of the entire New York State Barge Canal as a NHL in 2017.

Maggie L. Walker National Historic Site: The Maggie L. Walker National Historic Site is a United States National Historic Landmark and a National Historic Site located at 110½ E. Leigh Street on "Quality Row" in the Jackson Ward neighborhood of Richmond, Virginia.  The site was designated a U.S. National Historic Landmark in 1975.  The National Historic Site was established in 1978 to tell the story of the life and work of Maggie L. Walker (1867-1934), the first woman to serve as president of a bank in the United States.  It was built by George W. Boyd, father of physician, Sarah Garland Boyd Jones.  The historic site protects the restored and originally furnished home of Walker.  Tours of the home are offered by National Park Service rangers.

Ninety Six National Historic Site: Ninety Six National Historic Site, also known as Old Ninety Six and Star Fort, is a United States National Historic Site located about 60 miles (96 kilometers) south of Greenville, South Carolina.  The historic site was listed on the National Register in 1969, declared to be a National Historic Landmark in 1973, and established as a National Historic Site in 1976 to preserve the original site of Ninety Six, South Carolina, a small town established in the early 18th century.  It encompasses 1,022 acres of property.

Fort Vancouver National Historic Site: Fort Vancouver National Historic Site is a United States National Historic Site located in the states of Washington and Oregon.  The National Historic Site consists of two units, one located on the site of Fort Vancouver in modern-day Vancouver, Washington; the other being the former residence of John McLoughlin in Oregon City, Oregon.  The two sites were separately given national historic designation in the 1940s.  The Fort Vancouver unit was designated a National Historic Site in 1961, and was combined with the McLoughlin House into a unit in 2003.

Bok Tower Gardens: Bok Tower Gardens (also known as Bok Mountain Lake Sanctuary and Singing Tower ) is a National Historic Landmark, contemplative garden, and bird sanctuary located north of Lake Wales, Florida, United States.  It consists of a 250 acre garden, the 205 ft tall "Singing Tower" with its carillon bells, Pine Ridge Trail, Pinewood Estate, and a visitor center.  The tower is built upon Iron Mountain, one of the highest points of peninsular Florida, estimated to be 295 ft above sea level.  It is a National Historic Landmark that is listed on the National Register of Historic Places.

Supporting paragraphs: El Retiro (Lake Wales, Florida) Bok Tower Gardens
Answer: 250