WordPress REST API validation

There is no such maxLength option in the WP REST API.

You can pass a validate_callback though. Example:

<?php
add_action( 'rest_api_init', function () {
    register_rest_route( 'myplugin/v1', '/author/(?P<id>\d+)', array(
        'methods' => 'GET',
        'callback' => 'my_awesome_func',
        'args' => array(
            'id' => array(
                'validate_callback' => function($param, $request, $key) {
                    return is_numeric( $param );
                }
            ),
        ),
    ) );
} );

Source: https://developer.wordpress.org/rest-api/extending-the-rest-api/adding-custom-endpoints/