I'm on a notebook, and does not have a real estate hard disk.
So this is a minimalist partitions I made.
hda1 = 2 MB, for windows (I dual boot this machine)
hda2 = 512 MB, linux swap. The formula for swap is 2 x RAM, or max 512 MB. The tip is to put swap on the front of hard disk for better performance.
hda3 = 500 MB, linux /. This would be enough for /boot, /etc, /home (personal), /tmp, /var. From my experience, keeping this partitions small would increase performance (mainly because of the temp).
hda4 = extended partition (the rest of the disk, all of the rest partitions will be inside this)
hda5 = 1 GB, DOS drive D:. I also mount this from linux for shared data
hda6 = 2 GB, /usr. This will contains the linux applications. So far, it have been used up to 60%.
hda7 = 1 GB, /opt. Slackware puts /kde on this directory. I also puts non standard apps here (like Java).
hda8 = not used.
Thats all for personal use.
If you want to use it for server, you may want to make another partitions for /var (since it holds the logs, and usually http data that grows).
Totally you can play full slackware with only 5 GB.
How about the rest of the harddisk ?
Maybe you can use it for other distribution or BSD
Just expand the hda8.