nsl-username-length.php

<?php
/*
Plugin Name: Nextend Social Login - Username length
*/

add_filter('nsl_validate_username', function ($isValid, $username, $errors) {
    if (strlen($username) < 6) {
        $errors->add('invalid_username', '<strong>' . __('ERROR') . '</strong>:' . __('Sorry, username must contain at least 6 characters.'));
        $isValid = false;
    }

    return $isValid;
}, 10, 3);