noun
shirokuban (paper size 127×188mm); duodecimo
A traditional Japanese paper size, approximately 127mm by 188mm, used for books and documents. The term 'duodecimo' is a rough Western equivalent.
この本は四六判で印刷されている。
This book is printed in shirokuban size.
The name 四六判 (shirokuban) literally means 'four-six size', referring to the original sheet dimensions in 寸 (sun), a traditional Japanese unit of length. The exact derivation is uncertain, but it is a conventional term for this paper format.