1) Registration is done in Wordpress if you use Wordpress user integration mode. (Wordpress regularily cahnge their user database structure and format so we cannot add users to wordpress in a safe manner). Direct your users to register at http://yoursite.tld/wp-register.php page. You can add a link below the chat if you want to your registration page.
2) The chat width/height cannot be adjusted. I prefer to have the chat in a popup anyway, since the chat doesn't have to reload when someone is both reading your news/blog/articles as well as chatting at the same time.