B5 is about the same size as 16 kHz. These are two different sizes.

16 paper opening is 260mm* 184mm, and B5 is 250mm* 176mm.

Strictly speaking, B5 and 16 are code names of paper specifications, which are different in size, but very close. Due to the popularization and development of A4 paper, 16 gradually faded out of the market.