Completely untrue; Carl's wrong. The "common size" you're talking about is format-the number of times a sheet is folded to make one gathering. Folio is one fold (2 leaves, 4 pages), quarto is two folds (4 leaves, eight pages), and so one. But format does not determine the size of a book! The size of the paper sheet also matters, and sheet sizes have varied throughout time. The kindle is not a sheepskin folded over three times (which would be called an "octavo.") "Octavo" may mean something specific in contemporary book parlance, but it has absolutely nothing to do with medieval vellum.