Thank you for the opportunity to help you with your question! Hope your day is going well!
According to the websites found on google.com, the minimum password length for a domain controller is 7 characters long.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCEQFjABahUKEwiIlK_NkoLIAhXH8z4KHRj7Ab4&url=https%3A%2F%2Ftechnet.microsoft.com%2Fen-us%2Flibrary%2Fhh994560.aspx&usg=AFQjCNEG6vUlG7jIHJRylGgabHC7Ge1USw&sig2=tvrg5rHHN9zylG8UcyRfQg&bvm=bv.103073922,d.cWw <--where I found it.
Please let me know if you need any clarification. I'm always happy to answer your questions.