Web fiction
Bunkobon
Japan's small-format mass-market paperback, almost always A6 (105 by 148mm), used for affordable editions of previously published books.
In Japan, bunkobon are "small-format paperback books, designed to be affordable and space-saving"; the great majority are A6, 105 by 148 millimeters. The format is named for Iwanami Shoten's Iwanami Bunko library, launched in 1927, and like Western mass-market paperbacks it is typically used for cheaper editions of books that have already been published in more expensive form.
The trim matters to web-fiction writers because it is the standard size of the light novel: a serialized story collected into a print edition usually lands in this compact form. It is also the physical basis of Kanonara's fixed page, covered in the A6 bunko page format.