Retrieving actual file size on disk

Question

I am trying to retrieve the acutal size of a file on disk.  Similar to what windows does when you right click a file to view its properties and there is a field "Size on disk:".  I want to retriev that data.  I am not trying to retrieve data through the FileInfo.Length approach.  This is the size of the file when it is loaded into memory.  I need the size of the file in relation to the drive itself.  Can anyone help?