Retrieving Actual Disk Size with VBScript

I currently have a script written that retrieves the amount of space being used on a drive.  It shows the "Size" but not the "Size on Disk".  When you right click on a folder or drive and say properties it shows the "Size on Disk" which is the accuracy that I need for my script.  But I can't find how to retrieve this info using vbscript.  Does anybody know any links to sample code that does this.  Any suggestions appreciated.