The size is not about the actual physical size but about the bibliographical format. A book is not printed page by page of leaf by leaf. Several pages are printed together on a sheet of paper. Folded once a sheet would contain 4 pages and called in-2; folded twice it would contain 8 pages and called in-4; folded three times it would have 16 pages and called in-8. The number of possible ways to fold a sheet is enormous and some specialist books have been written on it. The most common formats were in-4, in-8 and in-12. In-2 was for luxurious books, the smaller formats were used for specific genres: cheap editions of the classics and songbooks.
Title page from an oblong edition of an emblembook.
There are two ways to look at front matter, body matter and end matter. A bibliographer would look at the way a the quires are signed and thus decide on bibliographical evidence which is which. But you can also look at the content and divide the different parts according to their content. The body contains the actual text - and of course the paratext that is directly related to it: notes, remarks in the margins, headers and footers. Front matter and end matter contain paratext with different functions: identification, recommendation, descroptions of the content, indexes and so on.
