Various documentation that I read said that the root of Ubuntu needed less than 200Mb. However, I've found that not to be true. In my situation, I ran out of disk space. So I've bumped the root partition to 500Mb. On my 80Gb drive, Here are my partition sizes:
medined@thog:~$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hda1               481764    282766    173296  63% /
/dev/hda8             51675672    923812  48126844   2% /data
/dev/hda5              3099260   1593760   1348068  55% /usr
/dev/hda7             10317828    556548   9237164   6% /usr/local
/dev/hda6             10317828    337728   9455984   4% /var

I also have a 1,000Gb swap - mainly because when I tried to install Oracle it asked for a 750Mb swap file.

UPDATE: One mistake (there are probably others) that I made was that the /tmp path needs a lot of room because that is where installation place files. So two choices - either give the root partition another couple of gigabytes just for software installation. Or give /tmp its own 2Gb partition. Or, as I am doing, constantly tell each installation process to place temp files in a /data/tmp (or whatever) directory.