If you are installing Windows 8 on Virtual Box you should know more about the Windows 8 hardware and disk space requirements.
Microsoft recommends 23035MB for a Windows 8 installation. Therefore we suggest to assign at least 24GB to a virtual machine.
However, this is only a pre-beta version, so this can change for the beta and the final RTM version.
We will keep you posted on the recommended disk space for other Windows 8 versions.