Treehouse - span and timber sizing help

Hi guys I am looking to build a treehouse between 2 trees with a tri-beam on each tree. The trees are spaced about 4.5-5m

The tribeams would be approx 4m in width each

I want to run 3 bearers between the tri beams which would span the 4-5m distance. - these would be sitting on top of the tri beams.

and then run joists at 450 centres across bearers - these would also be sitting on the bearers, no hangers.

Here is a design which I am aiming for something similar, with the third bearer in the middle. - tribeams will be connected via 2 TAB bolts in each.

1) Wondering what sizing the joists and bearers should be. The treehouse would also have a roof and enclosed area.
2) is 200x75 or 200x100 over kill for the tribeams? i dont think there is a span table to help me with those.

Thank you.