[Design of Marci Letter]

[ReneZ]

AFAIK, "[in] folio", "quarto" and "octavo" referred to how many times the original sheet of paper was folded and cut in half.  Folio (illogically) meant using the sheet without cutting, as one bifolio. Quarto means cutting it in half to make two bifolios = 4 folios. Octavo meant cutting twice to make 8 folios.

Yes, but parchment books had been made from hides before that for many centuries, and hides came in non-normalised (but also non-uniform) sizes. The reason I showed the plot is that there is almost a continuum of sizes even in this small sample of herbal books. Your plot also shows that clearly.
By using a colour code for the F/Q/O one could see how they work out.
However, I did not record these assignments (which are made by the libraries) so this would be a bit of work.

(I have seen codes 2o and 12o in some library catalogues as well.)