There's an explanation in Mark Forsyth's excellent The Etymologicon claiming it's based on the size of sheepskin. http://books.google.co.uk/books?id=KUZxTvay3P

There's an explanation in Mark Forsyth's excellent The Etymologicon claiming it's based on the size of sheepskin. http://books.google.co.uk/books?id=KUZxTvay3PMC&lpg=PP1&pg=PT50#v=onepage&q=papyrus&f=false http://books.google.co.uk/books?id=KUZxTvay3PMC&lpg=PP1&... (I can't seem to get a direct link to the page, but just click page).