Used as ssize action for multilog, as set in various /var/qmail/supervise/<module>/log/run files.
The default Qmail-Toaster setting is 1000000 (~1M).
man multilog excerpt:
ssize sets the maximum file size for subsequent dir actions. multilog will decide that current is big enough if current has size bytes. (multilog will also decide that current is big enough if it sees a newline within 2000 bytes of the maximum file size; it tries to finish log files at line boundaries.) size must be between 4096 and 16777215. The default maximum file size is 99999.