object.Size 

function ShowFolderSize(filespec)
{
   var fso, f, s;
   fso = new ActiveXObject("Scripting.FileSystemObject");
   f = fso.GetFolder(filespec);
   s = f.Name + " uses " + f.size + " bytes.";
   return(s);
}

 Function ShowFolderSize(filespec)
   Dim fso, f, s
   Set fso = CreateObject("Scripting.FileSystemObject")
   Set f = fso.GetFolder(filespec)
   s = UCase(f.Name) & " uses " & f.size & " bytes."
   ShowFolderSize = s
End Function