What you want to do is setup quotas. It is added to the /etc/fstab so it works on a specific partition. So if you have a partition like /dev/hda* is mounted as /home then you can apply quotas to the /dev/hda* which is /home. But if /home is part of the / partition then it will not work this way. I hope you understand this.
Now a way around this is create a partition like /samba. Then add to fstab with quota setup. Now create a directroy of each user there and that they have ownership to thier directory. Then create a softlink to that partition to thier home directory. ' ln -s /samba/user1 /home/user1/samba '.
So look up quota here, at www.linuxquestions.com
, and at google and learn about it.
Check out this post. Check the links in the second post on a virtual quota partition. Never tried but looked pretty good way without repartition.