A5 Size. Letter size paper is 8.5 x 11 inches (215.9 x 279.4 mm), whereas A5 size paper is 5.8 x 8.3 inches (148 x 210 mm). A5 has a wide variety of uses. It's commonly used for booklets, pamphlets, diaries, journals and planners, flyers, and it's a popular choice for paperback novels.