The width of the float not being fixed seems like a massive goal post shift. Unnecessarily so, honestly.

That said, this problem doesn't seem that much harder, honestly. Each float has a max size. They are only smaller if they had less text.

And see the example in the TeX-book. Basically a circular cutout for text aligned on a circle. Both parts fully hyphenated to fill the outline. I'm on my phone, but will try and find an example, if it would help.