pocket paperback size; bunkoban format
Refers to the standard small paperback size used for 文庫本 (bunkobon), roughly A6 dimensions (105×148 mm).
See also: 文庫本
This novel was reissued in bunkoban format.
Bunkoban-sized books are easy to carry around.
Compound of 文庫 (bunko, 'library' or 'book collection') and 判 (ban, 'size' or 'format').