Hello.
In these cases you will need to modify code-behind files and ensure requested functionality by a customization.
For
Change password web part, you can do it in
\CMSWebParts\Membership\Profile\ChangePassword.ascx.cs, in
btnOk_Click method. Here you can check length of password like this:
if (txtNewPassword.Text.Length > X)
{
}
The same can be done for
Site Manager/Administration/Users/Password page. It uses
CMSSiteManager/Administration/Users/User_Edit_Password.aspx page, and you can check it in
ButtonSetPassword_Click method.
Best Regards,
Radek Macalik