In the documentaion http://contactform7.com/text-fields/#textarea
[textarea* message id:contact-message 10x2 placeholder "Your Message"]
The above will generate a textarea with cols="10" and rows="2"
<textarea name="message" cols="10" rows="2" class="wpcf7-form-control wpcf7-textarea wpcf7-validates-as-required" id="contact-message" aria-required="true" aria-invalid="false" placeholder="Your Message"></textarea>