The airship hangar at Tillamook, Oregon has about 28,000 square meters of floor space making it about a third larger than the Tacoma dome and about ten times the size of the Great Buddha Hall.  I believe the airship hangars at Tustin, California are of similar dimensions to Tillamook.  They stand a thousand feet long, nearly 300 feet wide and nearly 200 feet tall, and inside it's all clear space.  They are clearly buildingsthey have roofs and walls and are completely enclosed from the elementsand while they use metal fasteners they are of the sort of construction that most people think of as "wooden" so I can't think of a reason why one of them should not be listed as the largest wooden building. 
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