max-length.php

<?php

/**
 * Modify max length of first name field in WooCommerce.
 *
 * @param array $fields
 *
 * @return array
 */
function flux_modify_fname_field_length( $fields ) {
	if ( isset( $fields['billing']['billing_first_name'] ) ) {
		$fields['billing']['billing_first_name']['maxlength'] = 20;
	}

	if ( isset( $fields['shipping']['shipping_first_name'] ) ) {
		$fields['shipping']['shipping_first_name']['maxlength'] = 20;
	}

	return $fields;
}

add_filter( 'woocommerce_checkout_fields', 'flux_modify_fname_field_length', 1001 );