We don't have a parameter for adjusting the length of the randomly generated passwords. They'll always generate with a length of 12 characters. As for the characters they're entirely random as per Joomlas function for generating random passwords. Those parameters, except for length, have any impact on how Joomla generates random passwords. We do provide a onUserRandomPassword trigger if you want to completely override how random passwords are generated though and is the only way to adjust length.