Code:
alias fsize {
  var %a = fsize $+ $ticks, %b = %a $+ file, %c
  .comopen %a Scripting.FileSystemObject
  if ($comerr) return
  noop $com(%a,GetFile,1,bstr,$1,dispatch* %b)
  if $com(%b) {
    if ($com(%b,Size,2)) %c = $com(%b).result
    .comclose %b
  }
  .comclose %a
  return %c
}

The above can be used without checking if the file exists first with $isfilew: if it doesn't, $fsize returns $null.