noun
B6 paper size (128 mm × 182 mm)
Standard paper size in the Japanese B-series, commonly used for books, notebooks, and documents.
この手帳はＢ６判で、持ち運びに便利です。
This notebook is B6 size, convenient for carrying around.
Ｂ６判の原稿用紙に清書してください。
Please make a fair copy on B6-size manuscript paper.
A5 is slightly larger (148 mm × 210 mm) and also common for notebooks and booklets.
B5 is larger (182 mm × 257 mm), often used for textbooks and magazines.
From the B-series paper size standard (ISO 216), where 'B6' indicates the specific dimensions.