Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
  If TypeOf Item Is Outlook.MailItem Then
    Cancel = Not (ConfirmBigAttachments(Item))
  End If
End Sub

Private Function ConfirmBigAttachments(oMail As Outlook.MailItem) As Boolean
  Dim lSize As Long
  Const MAX_ITEM_SIZE As Long = 10000 ' Byte
  Dim bSend As Boolean

  bSend = True
  If oMail.Attachments.Count Then
    oMail.Save
    lSize = oMail.Size
    If lSize > MAX_ITEM_SIZE Then
'      bSend = (MsgBox("Größe: " & lSize & " Byte. Abbrechen?", vbYesNo) = vbNo)
      bSend = (MsgBox("Item's size: " & lSize & " Byte. Cancel?", vbYesNo) = vbNo)
    End If
  End If
  ConfirmBigAttachments = bSend
End Function