What is the smallest (installed size) ubuntu based distribution?

Try Ubuntu minimal ISO from here.

Choose the packages you want to install and you are done. You can try using LXDE or Openbox Desktop environment as they are not heavy on resources.

If you want just a simple Web browser and few cloud based services try an OS called xPUD.

xpud.org

8GB is not a small size. You can use Ubuntu with a 8GB hard disk. But If you are still looking for a more lightweight Ubuntu based distro check this out. I am not sure which ones among them are still active but I suggest those personally.

• Lubuntu which runs with LXDE desktop.
• Bodhi Linux which runs with Enlightenment window manager.
• CrunchBang++ Ben Young's continuation to Crunchbang Linux. Debian + Openbox
• BunsenLabs Linux Community continuation to Crunchbang Linux. Debian + Openbox

Remove /usr/share/doc or something like that. I've never used help files for graphical applications, so I've cleaned about 400MB of disc space. :) (I use Kubuntu 11.04)