How to show HTML5 Export Buttons and Length Drop Down at the same time? (DataTables 1.10.9)

I have the code:

$('#example').dataTable({
                   dom: 'Bfrtip', 
                   buttons: [
                         {
                             extend: 'excelHtml5', 
                             extension: '.xlsx',
                             text: 'Excel'
                         }
                     ] 
      });

And now I have a button, but no Lenght anymore. How can I also add Length Drop Down to my layout and keep my Export button?