[Design of Marci Letter]

[Jorge_Stolfi]

Still, there was no standard. Folio, Quarto, Octavo can be considered as groupings.

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.

I would expect a series of standard sizes to develop spontaneously within a market area, like it did for gold and silver (coins), barrels of wine, etc.  Not only to simplify trade, but also so that a consumer who is writing logbooks or some other series of documents would not have to trim every sheet, or every new batch, to fit the size of earlier docs.  

All the best, --stolfi