Private Sub CmdPrint_Click(ByVal sender As Object, ByVal e As _
      System.Windows.RoutedEventArgs) Handles CmdPrint.Click
        Dim PrintDlg As New PrintDialog
        'Dim PageSize As Size = New Size(PrintDlg.PrintableAreaWidth, 
        ' PrintDlg.PrintableAreaHeight)
        'Me.FlowDocPreview.Measure(PageSize)
        'Me.FlowDocPreview.Arrange(New Rect(5, 5, PageSize.Width, 
        ' PageSize.Height))
 
        If PrintDlg.ShowDialog = True Then
            Dim dp As DocumentPaginator = DirectCast( _
              Me.FlowDocPreview.Document, _
              IDocumentPaginatorSource).DocumentPaginator
            dp.PageSize = New Size(96 * 8, 96 * 11)
            PrintDlg.PrintDocument(dp, "TEST")
        End If
    End Sub

Jedes Programm,
in das sich ein Fehler einschleichen kann,wird auch einen enthalten.