Comment 2 for bug 163541

No, what it actually means is half of the remaining space available after accounting for the fact that you've got to get an Ubuntu installation in there too. The current fudge factor for this is about 2.21GB; (7.78-2.21)/2 is 2.785.

This should all be a lot clearer in Hardy, now that Evan has turned the auto-resize UI into a slider.