Minimum Password Length is a simple plugin that requires user passwords to meet a predefined length requirement.
The default length is 7 characters. To change this, simply edit the plugin file and change the value of $min_length to the desired password length. This is, admittedly, a little clunky, but I didn’t think a single setting warranted a new settings page.
Note: If you want to control the strength of user passwords, check out my other plugin Minimum Password Strength