Here, you can configure the size of the history buffer, i.e. the
number of log lines that is kept. If more lines have been emitted by
the GnuPG backend since the last clearance of the history, then the
oldest lines are discarded until there is enough room for the new
lines again.
You can disable the history size limit by clicking Set
Unlimited. Note, however, that KWatchGnuPG's memory
consumption will grow with the number of lines it currently
displays. If you use an unlimited history size, then make sure to run
KWatchGnuPG only for short operations, or regularly clear the
history manually.