I'm doing a custom backup, it's even less than the recommended though, so the size is puzzling. I have it set to backup "Backup Data for Newly Created Users," documents,pictures, and a system image.
As for the estimated backup size, I might be losing my mind, I can't find it now. I think I just imagined it said that above the shield and "Manage Space" button.
Right now in my D drive there are 4 files displayed(with the proper boxes unchecked, displaying hidden files and OS files.)
$RECYCLE.BIN - Inaccessible, access denied.
Erik-PC
System Volume Information
MediaID.bin - Inaccessible, access denied.