Oh, as for them not being necessary beyond a certain wingspan, I'd say "it depends". If you use overly-large structural members for things like the wing spar, then you may not need them. But if you are using the same size wood for spars in a 20" wing as you are on a 40" wing, you're using too much lumber to begin with. Also, most 20" planes that I've seen have been indoor or park flyer models that don't encounter high g-forces, meaning you can use a lighter structure, possibly leaving out shear webs.

My old Supra-fly .25 size (designed by Hano Pretner, back in the 80's) was built really light, and flew great. 48" wing. The outter 3 bays didn't have shear webbing. Out there, the bending loads are very light, so it wasn't necessary. The spars were also very thin, but were doubled up for the inner third of the wing, basically creating a tapered spar. Good design puts structure where you need it and nothing where you don't.