As far as I remember, all default versions of PM add-on contain the same posting form, as it's present under the templates/main_post_form.html
. Since version 2.2, textarea of this form contains the following CSS attributes:
<textarea name="postText" id="postText" class="textForm postingForm" tabindex="2"></textarea>postingForm
class is exactly the one which should be present under your CSS and which defines this form's width by default.
I guess, if it's there, then it's not required to specify a generic CSS class for all textarea's by default.