Your Twitter username must be at least four characters long to be valid. They also have a maximum length of 15 characters. In addition, you can’t use a username that is already being used by someone else, and, as mentioned above, it can only contain alphanumeric characters or underscores. Display names, on the other hand, can be a single character if you wish and have a maximum length of 50 characters. Again, as mentioned above, symbols and emojis can be used on your display name, just not the username/Twitter handle.