While there are varying opinions on this subject, most collectors of miniature books define them as not exceeding 3” (or 76 millimeters) in either height or width. Early manuscripts, traveling libraries and some early European texts are deviations from this rule. Microminiatures measure less than an inch and are sometimes referred to as dollhouse books.
The example images show microminiature and miniature books. The first shows the scale of a book, the second shows a book with a metal clasp, the third shows a book with leather covers and a belly band to hold it closed, and the fourth shows both miniature and microminiature sizes.