The mdbox_rotate_size and mdbox_rotate_min_size parameters are
specified in bytes not kilobytes.

doc/example-config/conf.d/mail.conf:

Maximum dbox file size in kilobytes until it's rotated.

#mdbox_rotate_size = 2048

Minimum dbox file size in kilobytes before it's rotated

(overrides mdbox_rotate_days)

#mdbox_rotate_min_size = 16

src/lib-storage/index/dbox-multi/mdbox-settings.c: MEMBER(mdbox_rotate_size) 2048*1024, MEMBER(mdbox_rotate_min_size) 16*1024,