First of all, just a hint: if your user is not providing HTML themselves, there is no need to use qa_sanitize_html(), since there's no way for the user to inject Javascript or other nasties. Certainly you shouldn't need it for HTML that you build yourself in your viewer.
As for your question about the textarea, I'm afraid it's not currently possible. You do of course have the option of returning a Q2A form element that describes a textarea, but this won't allow you to fully customize the HTML around it.
So for practical purposes I think you're fairly safe just hard-coding CLASS="qa-form-tall-text" (and whatever else is appropriate) in your HTML. It's true that someone's custom theme could render that CSS class meaningless, but in practice it's not very likely to happen.