Answers for "datatable change default page size"

datatables change width of columns

$('#example').dataTable( {
  "autoWidth": false, // might need this
  "columns": [
    { "width": "20%" },
    null, // automatically calculates
    null  // remaining width
  ]
} );

datatable change default page size

Javascript
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$('#example').dataTable( {
  "pageLength": 50
} );

Datatable Change page size for pdf

buttons: [
            'csv', 'excel',
            {
                extend: 'pdfHtml5',
                pageSize: 'A3',
                exportOptions: {
                    columns:':visible',
                    rows: ':visible'
                }
            },
        ],