How should I partition the hard drive prior to installing Plesk ?

We cannot give any exact numbers, however, you should take into account the following.

Plesk for Linux OSes stores all users' data in /var partition by default, so the partitioning should be like:

SWAP -> twice RAM size
/boot -> 128 MB
/ -> System + Plesk, updates. 5-10 GB should be enough.
/tmp -> 1GB.
/var -> the rest of the space (domains, mail, backups, databases are stored there)

Directories that usually take much disk space:

/var/www/vhosts (/srv/vhosts on SuSE) - domains.
/var/qmail - mail.
/var/lib/mysql - Mysql databases.
/var/lib/psa/dumps - Plesk clients'/domains' backups.
/var/lib/pgsql/data - Postgres databases.
/var/tomcat* - Tomcat applications.
/var/lib/mailman - Mailman lists.

Please check file /etc/psa/psa.conf for all other directories used by Plesk control panel.

Plesk Contorl Panel under FreeBSD uses the /usr/local partition for storing large quantity of data and /var/db partition for databases.
So the /usr and /var partitions have to be larger in this case.

On Debian and Ubuntu, Plesk itself is located in /opt/psa.