Approximately 1,500 km^2 across Poland and Belarus

Photo from Wikimedia Commons
Bialowieza Forest
Europe's last primeval lowland forest, bison included/Forests
Bialowieza Forest straddles the border of Poland and Belarus, covering roughly 1,500 square kilometers of ancient woodland that has remained continuously forested for over 8,000 years. It is the last significant remnant of the primeval forest that once blanketed the European Plain. The forest is home to the European bison, which was saved from extinction here, and contains oak trees over 500 years old reaching 40 meters in height. Walking through Bialowieza feels like visiting what all of Europe used to look like before people decided they needed more farmland.
Measurements
Oldest oaks, over 500 years
Over 8,000 years
Largest free-roaming herd in the world
Including oak, spruce, pine, and hornbeam