Teardrops n Tiny Travel Trailers

Mine is 6x13 with a flat plywood roof, supported by 1x3 spars on edge. I keep the tear parked nose or tail high for drainage. I plan on creating a standby out of it someday, with a bowed roof to not have to think about draining

I would think that if you sandwich a wood "stringer"( like they do with surfboards) you will probably be ok.
https://youtu.be/qNHWbRV432s

Trick question.
They're building foamie houses and buildings.
If you include SIPS panels.
Basically, like any living thing, reinforcing structure frame required the bigger one builds.
Foam only seems 4x4x8 2 inch XPS is reasonable unsupported or unframed if that is what you're getting at.
Larger if skinned on both sides.