Comment 10 for bug 745148

I've increased the minimum size to double the size of the install, or roughly 5.2 GB. This is just above the minimum size set by the official documentation:
https://help.ubuntu.com/10.04/installation-guide/i386/minimum-hardware-reqts.html

Note that this is a hard minimum. While installs below this size are technical possible, they are not allowed by the UI.