Approximately 68,700 km^2

Photo from Wikimedia Commons
Tongass National Forest
America's largest national forest, bigger than some countries/Forests
The Tongass National Forest in southeastern Alaska is the largest national forest in the United States, covering roughly 68,700 square kilometers of temperate rainforest across the Alexander Archipelago and mainland coast. It contains old-growth trees over 800 years old, some of which are Sitka spruce reaching 60 meters tall. The forest receives up to 4 meters of rainfall annually and is home to brown bears, bald eagles, and all five species of Pacific salmon. It is essentially Alaska's version of a tropical rainforest, minus the tropics.
Measurements
Up to 4 meters in some areas
About 18,000 km of shoreline
Approximately 800 years