Function ThisWorkbookFolderSize() As Long
Dim objFSO As Object
Set objFSO = CreateObject("Scripting.FileSystemObject")
'This will cause an error if the workbook is not saved
Set objFSO = objFSO.GetFolder(ThisWorkbook.Path)
ThisWorkbookFolderSize = objFSO.Size
Set objFSO = Nothing
End Function