Unfortunately, the comment box can't be made responsive at this time. That was possible, and JFBConnect supported it, until mid-March of this year. At that time, Facebook implemented a hard-coded width on the box that could not be overridden in any way. You can see the full bug report below in which many other users are unhappy with the change as well:
developers.facebook.com/bugs/256568534516879/
The bug has been 'assigned', and we're hopeful it will be fixed soon. When it's fixed, if there's anything we need to implement in JFBConnect, we absolutely will. We liked the responsive box as well.
I hope that helps explain, even if it's not what you were looking to hear.
Thanks,
Alex