Julien,

> I am interested in seeing the interface and implementation, given that I
> wrote something similar (but probably less thorough since mine is
> essentially wrapping NFSS/latex2e)

Here is the module source:
https://github.com/tail-reversion/soranus/blob/master/fontsize.tex

There /was/ user documentation for this module, but a mistake on my part means
that said documentation has ``disappeared''. Hopefully, the source is clear
enough. Please excuse the mess of a repository. And I plan eventually to switch
over to dtx files, but presently I find it easier not to fiddle with docstrip.

I like the idea of making font sizes relative to the main document size. I had
actually experimented with exposing the normal size as a public variable for
use in instance definitions, but I found the dimension scaling to be of
unsatisfactory precision. However, I just learned that the l3kernel provides
floats of higher-precision than TeX dimensions, so perhaps that could be a
solution.

-Kelly