How are you implementing the comment box? Is it from our content plugin, configured in the JFBC -> Social area, an instance of the SC Social Widget module, or through the {JFBCComments} Easy Tag?
In either case, you can set the width to 100% and that will make it responsive to the widget its in. Hopefully, you can do that wherever you configured it, but if not, let us know what's implementing the comments, and we'll tell you how to set it up.
Thanks,
Alex