Called by an application that is administering the device to set the minimum allowed password length. After setting this, the user will not be able to enter a new password that is not at least as restrictive as what has been set. Note that the current password will remain until the user has set a new one, so the change does not take place immediately. To prompt the user for a new password, use DevicePolicyManager.ActionSetNewPassword after setting this value. This constraint is only imposed if the administrator has also requested either DevicePolicyManager.PASSWORD_QUALITY_NUMERIC, DevicePolicyManager.PASSWORD_QUALITY_NUMERIC_COMPLEX, DevicePolicyManager.PASSWORD_QUALITY_ALPHABETIC, DevicePolicyManager.PASSWORD_QUALITY_ALPHANUMERIC, or DevicePolicyManager.PASSWORD_QUALITY_COMPLEX with DevicePolicyManager.SetPasswordQuality(Android.Content.ComponentName, Android.App.Admin.PasswordQuality).

The calling device admin must have requested DeviceAdminInfo.UsesPolicyLimitPassword to be able to call this method; if it has not, a security exception will be thrown.

[Android Documentation]