This requires modifying 2 files:
in core EE tpc/dompdf/include/cpdf_adapter.cls.php on line 139, add a line so that it then reads:
"11x17" => array(0,0,792.00, 1224.00), "ticket" => array(0,0,144,396) );
then in the ticketing add-on, in functions.php on line 244, change it to be:
You will have to produce a new ticketing template and css file, probably from scratch, in order to lay out the ticket to match the new size.