[For semi-submerged spheres], the part of the sphere that emerges from the water has a curvature. Once that curvature goes a bit vertical, it’s no longer useful for occupation by people. Just to take a WAG at where it starts to be a trade-off, most human activities don’t take place on a surface more than 30 degree or so- that’s where (for example) vineyards start to become impractical for machinery to navigate and for people to stand and do manual labor. It takes a pretty large sphere to appear flat- even the Earth’s curvature is observable from a moderately tall structure like a tall sailing mast.
I should have said, “A sphere also has a minimum size where the emergent disc is useful.”