The actual size will depend on many variables that the user selects on the installer. Some suggest the minimum space required is closer to 9GB. With everything included, including X11 files, it approaches more like 15GB. While the "System" folder may be much less than either of those figures, you must remember that OS X has another critical folder/directory called "Library" (as does each user created) and that may be much larger than the "System" folder. I don't think checking the size of the "System" folder would include all the files in the root directories, either. This OS has stuff literally all over the place, much of it invisible to the Finder and the 'normal' user. Makes guestimating the size of the System installation much harder than in previous Apple OSs. Just another reason for not attempting an install on a drive with 'little' usable space. Some of the space requirements for the install may not be needed after the OS is installed. Moving duplicated items/files to temporary locations until their final location is created and the original is deleted, for example, will require some space that will be recovered. But the space is needed, in any case, for a successful install.