BuddyPress.org

That depends on where you’re using it. You could use a bp_signup_validate filter to check that a username has certain characteristics. But for profile fields or other places, you’d have to enforce it some other way.

Here’s an example of how you might disalloe long usernames:

add_action('bp_signup_validate', function() {
	if ( isset( $_POST[ 'signup_username' ] ) && strlen( $_POST[ 'signup_username' ] ) > 60 ) {
		$bp = buddypress();
		$bp->signup->errors['signup_username'] = 'Geez, your username is really long.';
	}
} );

You could put it in here:

https://codex.buddypress.org/themes/bp-custom-php/