There is not currently an option for this, but you can implement it by modifying the qa_question_validate(...) function in qa-app-post-create.php.
If you wanted a maximum of 200 characters, you would change this:
qa_length_validate($errors, 'title', $title, $options['min_len_q_title'], QA_DB_MAX_TITLE_LENGTH);
...to this...
qa_length_validate($errors, 'title', $title, $options['min_len_q_title'], 200);
An option regarding this is planned for the next major release.