Sorting attributes in custom order

functions.php

<?php

/**
 * Modify sorting pattern for size attribute.
 */
function swpco_weekdays() {
	/* Following data is assumption that you want the size order in mentioned order. */
	return array(
		's'    => 'aaaaaaaaa',
		'm'    => 'aaaaaaaab',
		'l'    => 'aaaaaaaac',
		'xl'   => 'aaaaaaaad',
		'xxl'  => 'aaaaaaaae',
		'xxxl' => 'aaaaaaaaf',
	);
}
add_filter( 'swpco_weekdays', 'swpco_weekdays', 10, 1 );

/**
 * Modify sorting type for size attribute.
 */
function return_type_of_attribute( $type ) {
	return 'weekday';
}
add_filter( 'swpco_attribute_type', 'return_type_of_attribute', 10, 1 );