Folio paper is 8.27 x 13 inches (= 210 x 330 mm).
That is the modern size, defined in metric units.
Books are sometimes described as being folio, referring to the size of each page. The traditional size, defined in imperial units, was 81/2 x 131/2 inches (216 x 343 mm ). An alternate name, foolscap, referred to the traditional watermark design, depicting a medieval fool's cap.