Below is a code snippet to resize the grid to fix only its parent container width. 

<div id="mydiv" style="width:600px">

<?php
$dg = new C_DataGrid("SELECT * FROM orders", "orderNumber", "orders");
$dg->enable_autowidth(true)->enable_autoheight(true);

// set width to parent DIV
$dg->before_script_end .= 'setTimeout(function(){$(window).bind("resize", function() {
        phpGrid_orders.setGridWidth($("#mydiv").width());
    }).trigger("resize");}, 0)';

$dg -> display();
?>

</div>