Quote from MS
When the "Install Windows 8" message appears, select "Install by creating media" to create a bootable USB flash drive or an .iso (a disk image) file.
Step 3: Wait for the Assistant to download the Windows 8 Pro installation package and create the bootable media or .iso file. The flash drive must be have 3GB or more of free space available.
Step 4: Burn the .iso file, if that's the path you chose, to a blank DVD.
end quote - sounds like about 3 gigs