How large should I make root, home, usr, var, and tmp partitions?

First, I would put all space, except /boot, in a LVM2 volume. That's because it makes it so much easier to adjust partition size later, even on live systems and modern file systems.

Then I would make root small about 1GB, /usr about 6GB, /var about 2GB, /boot about 512 MB, swap same size as RAM, /var/log, /home and /srv large enough. It can be adjusted later.

I would not used all unallocated space now, as that could later be used to expand the other partitions, as I'm using LVM2. I could even add new paritions like /tmp, but that is a bit more complicated. I never use anything but LVM2 anymore. With that, I can even move the installation to RAID disks in less than 30 minutes, including creating and copy files and all.

For more information about LVM2, read this: http://www.debian-administration.org/articles/410

Yes, about encryptions, please have a look at Luks support:

• http://www.hermann-uwe.de/taxonomy/term/95
• http://www.debian-administration.org/users/dkg/weblog/71
• (Package cryptsetup) file:/usr/share/doc/cryptsetup/README.remote.gz

There is no correct or wrong answer,personally i would make a 12GB partition for my Ubuntu installation containing:

-> 6GB for root
-> 2GB for swap
-> 4GB for home