The rules are 67 characters including the extension such as ".com", so actually the name has a limit of 63 characters (in fact, the domain name checker will only allow you to use 63 * characters).
Please note that only 0-9, a-z, and the dash "-" are valid characters.

You may lookup your domain names here
http://www.clients.mytruehost.com/cart.php?a=add&domain=register