Here is the output of my slackware 12.2 machine.
Filesystem Size Used Avail Use% Mounted on
/dev/root 973M 264M 710M 28% /
/dev/sda1 95M 55M 40M 59% /boot
/dev/sda6 487M 74M 413M 16% /var
/dev/sda7 4.8G 345M 4.5G 8% /tmp
/dev/sda8 39G 12G 27G 30% /usr
/dev/sda9 4.8G 926M 3.9G 19% /home
/dev/sda10 648G 111G 538G 18% /library
I have been using this machine as storage and internet. Openoffice is installed to /opt which is bind mounted to /usr/opt. /opt generally doesn't get much attention. Here are the directory sizes for somethings that go there some times.
# contains only openoffice 3.0 on my machine.
du -hs /opt
# some other things
# this works on slackware
sed -n -e '/UNCOMPRESSED/p' /var/adm/packages/kde*
# Then you can add up the output sizes to find KDE in total is
# 436 MB
# Gnome would probably take up just as much.
du -hs /usr/lib/firefox-3.0.10/
du -hs /usr/lib/thunderbird-184.108.40.206/
du -hs /usr/lib/seamonkey-1.1.16/
So even in a worst case scenario, you are looking at 3 gigs for /opt with plenty of room to spare. Again, check with your distro, but I feel 7gigs on /opt is an extreme waste when it won't even be 50% used.
If you want to run an http or ftp server, just make sure you have things set up so that stored files are on your /home partition unless you don't plan on exceeding the size of /var which you have set at 7gigs. My /tmp is 5 gigs because of potential use and I build packages in /tmp and sometimes forget to delete them until reboot or it fills up.