Ummm ... for all I know slack defaults to
using logrotate, which should keep the
logs to a more weildy size ... and you can
then just remove older versions ...
Is logrotate installed on your box?
grep -l logrotate /var/log/packages/*
P.S.: For now it should be reasonably safe to
echo /dev/null > /var/log/syslog
(if you're sure there was no intrusion attempt :})