The message box adjusts automatically. I generated an email field tag (826) for Email. 33 is the size. You can play with that number to achieve the desired results.
<p>Your Email (required)
[email* your-email-826 33/] <p/>
I generated a text field tag (252) and used the same one for the name and subject. 33 is the size.
<p>Your Name (required)
[text* your-name-252 33/] </p>
<p>Subject
[text your-subject-252 33/] <p/>
Hope this works for you.