Biggest foamies?

[rowerwet]

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

[Socal Tom]

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

[Terra6]

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.