wcpauau_aspect_ratio.php

<?php
/**
 * Filter the aspect ratio by field idea
 * @param $ratio	Default ratio
 * @param $id 		The field ID in the form, pewc_group_1234_5678
 * @param $field  The field object
 */
function prefix_aspect_ratio( $ratio, $id, $field ) {

	// Check for our field ID here
	// Change 8888 to the ID of the field where you want to specify a different ratio
	if( $field['field_id'] == 8888 ) {
		$ratio = '16/9';
	} else if( $field['field_id'] == 9999 ) {
		$ratio = '2/3';
	}

	return $ratio;

}
add_filter( 'wcpauau_aspect_ratio', 'prefix_aspect_ratio', 10, 3 );