The Alnwick Garden in Northumberland County, UK, billed itself as the world's largest all-wood treehouse (although you may argue it's a house in the trees, not an actual treehouse):
In 2005, the world’s largest all-wood treehouse was built amongst the lime trees of the non-profit Alnwick Garden in Northumberland County, UK. This 6,000 square foot treehouse is comprised of walkways, cottages, shops, a restaurant, and play areas. It is even wheelchair accessible, and holds close to 1,000 people, so everyone can play.
Say No to Crack has more pics: Link