I'm building a wood 4x8 machine, and am using a square torsion box gantry, roughly 200mm x 200mm.
I'm using laminated MDF for the torsion box, because it was free.
Here are a couple links.

https://www.cnczone.com/forums/cnc-w...ml#post1162909

https://www.cnczone.com/forums/cnc-w...ml#post1168468