Default username length in the current version equals to 15 and it's specified in setup_options.php -> $uname_maxlength.
It's limited mostly for 2 reasons:
— short laconic usernames look good on the screen and don't heavily overly with the primary content;
— regarding default miniBB layout (incl. listings of topics and posts), long usernames won't fit it without breaking, this specially affects the Mobile layout which is designed mostly for vertical screens.
The login and registration forms are following the limitation set in options.
The length could be surely extended, but then you also have to take care of modifying the layout/templates, testing more longer usernames on your own.
Username is what is displayed on the screen as the forum alias, and by default, it's not possible to register emails as usernames in miniBB. This is specified in the username pattern of setup_options.php -> $userRegName.
Also, it's a bad idea to display email addresses to public as forum aliases.
However, in miniBB it's possible to login using the email address specified in the profile. On the login form, it could be entered in the Username field and using profile's Password. It's a default option and nothing should be specially configured for it. | 
