This particular method didn’t work for me on my main pages, but I found that using the id instead of class tag did work.
So, for example, I added #contact-form { width: 150px; } to my stylesheet. Then, within the plugin, I used the id rather than class tag. So, for example, my “first name” form field looks like this: [text* your-name id:contact-form akismet:author].