Bibliographers often describe books by their format — their size, shape, and construction — which gives important information about the way in which they were printed and bound.
Most codices are produced from a collection of sheets, folded to produce leaves and pages.
Books range from the large folio, through the quarto, octavo, and duodecimo, to the smaller and less common formats like 16mo, 24mo, 32mo, and the teensie-weensie 64mo.
A rough guide: modern unabridged dictionaries are usually the size of quartos; most textbooks are octavos; popular paperbacks are often duodecimos.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.