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Next create a new virtual drive for you Windows 8 VM. 20GBs should be more than enough for test purposes.
Unless you plan on using your virtual Windows 8 machine with about VM program, it's best to just stick with the VirtualBox Disk Image for your virtual hard drive.
Personally I prefer to use dynamically allocated virtual drives—just in case I start using a VM instance a lot and it expands beyond its original 20GBs of designated space.