[Hugh R]

I think I used 2"x 6" in retrospect I'd have gone larger, but its at most a six foot span. It hasn't sagged in ten years. Bolt into the tree through a piece of short 2" x 4" with the frame of the tree house bolted to the top of the short 2 x 4. Another thing you could do is use four posts and build around the tree and not attach to it.