Adding Excel export button hides/removes the Page Length Dropdown in Data Tables

Add below line in javascript table initialization

"dom": '<"top"Bf>rt<"bottom"lip><"clear">'

Complete Code 
<script>
        $(document).ready(function () {       
        $('#example').DataTable( {
                "processing": true,               
                "lengthMenu": [[25, 50, -1], [ 25, 50, "All"]],
                "dom": '<"top"Bf>rt<"bottom"lip><"clear">',
                "serverSide": true,
                "ajax": "server_processing.php"
            } );
        });                     

</script>