If you scale up the plans, what's the thickness of the resulting ribs?

What material you make them from depends a bit on how much you lighten them. Off the top of my head I'd think that something like 1/8" or 3/16" balsa or 1/8" lite ply would work. You could also use CF hollow spars or if you go with the FG tubes, basswood spars.

It might also be worth having a look at the large chaos plans sold by bluejay with the kit.

David