Book Size & Type
8vo - Octavo
Books can vary from miniatures (technically, books under 3 inches tall) to "elephant (or atlas) folios" (23-25 inches tall). Most books are just 8vo (pronounced "octavo" or "eight-vo"), i.e. roughly 8 to 9 inches tall. 8vo is the most common size for a book. Most fiction and poetry, most biographies and history books, etc. are this size.
4to - Quarto
Slightly larger books, measuring between 10 inches to just under 13 inches tall, are called 4to (pronounced "quarto"). Many children's books and picture books, including reference books, etc. are this size, and, like certain magazines and smaller atlases are just a little too tall to fit on many bookshelves next to the normal-size (8vo) books.
Folios
Books taller than 13 inches are called folios, and are often too big to sit upright, and have to lie on their sides on a top or bottom shelf, or on a table somewhere. These include what are often called "coffee table books" - just plain big.
Other Sizes
Books that are smaller than 8vo use larger numbers, so, for example, 12mo (usually pronounced "twelve-mo") describes a book about 7 inches tall; 16mo ("sixteen-mo") a book 6-7 inches tall; 24mo ("twenty-four-mo") one that is 5-6 inches; 32mo would be 4-5 inches; 48mo for under 4 inches; 64mo is about 3 inches, and, last but not least, miniatures, which are under 3 inches tall. All these "mo's" and "vo's" are Latin terminology for the number of times a large sheet (or "folio") has been folded. Since there are some variations in this original size, we need to use ranges rather than exact measurements for all these "mo's and vo's".
Some bookmen feel we'd all be better off just giving exact measurements for every book. That would be nice except for the fact that these terms have been around a long time and are pretty well established in use. They aren't that difficult to learn and mainly refer to size. To sum up, most books are 8vo, slightly larger ones are 4to, and big ones are folios. For the smaller sizes see the previous paragraph.