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 . 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.