DataTables::create(array(
    ...
    "options" => array(
        "columnDefs" => array(
            array(
                "width" => $yourColumnWidth, //in px, cm, or %
                "targets" => $yourColumnIndex
            )
        )
    )
));

I forgot that we must set DataTables' autoWidth to false as well:

DataTables::create(array(
    ...
    "options" => array(
        'autoWidth' => false,
        "columnDefs" => array(
            array(
                "width" => $yourColumnWidth, //in px, cm, or %
                "targets" => $yourColumnIndex
            )
        )
    )
));

It should also be noticed that if your other columns' combine min widths are large, your preferred width column won't be able to be exactly 300px. You could reduce the number of columns to see the effect of width setting. Thanks!