I have installed a lot of apps and my / (root) is about 6 to 7GB. My /home is only 1GB with .wine & Picasa taking up most of that. I have all my data in other partitions.
One place recommended to keep / at 25% use. I have typically recommended 10-20GB for root. Writing a DVD may use another 4GB in /tmp so I like to have room and have 20-25 GB for every root I have.
You may want to houseclean apps before making list. I still am reinstalling python 2.5 which I do not want anymore for example.
from lovinglinux - use dpkg to list installed apps
and using synaptic
File > Save Markings As, tick the "Save full state, not only changes". If you don't tick the 'full state', you will probably get an empty file.
To restore, you would use File, 'Read Markings'