4.24 square kilometers today

Photo from Wikimedia Commons
Sherwood Forest
Robin Hood's legendary woodland in Nottinghamshire, England/Forests
Sherwood Forest in Nottinghamshire, England, is famous as the legendary home of Robin Hood and his Merry Men. Once a vast royal hunting forest covering much of Nottinghamshire (perhaps 400 square kilometers in medieval times), the designated National Nature Reserve today covers about 4.24 square kilometers. It contains over 900 veteran oak trees, including the Major Oak, estimated to be 800-1,000 years old with a canopy spread of 28 m. The forest was a royal hunting ground from the Norman Conquest until the English Civil War.
Measurements
About 400 km2 in the Middle Ages
Supported by poles
Circumference of the famous tree
Shortened by age
Mature English oaks