Allowing a % width is something that Facebook just updated their Comment widget to allow (thankfully!). When they originally made the update, we tested with the {JFBCComments} tag and it worked. I haven't tried when using the Comment plugin, but looking at the code, entering "100%" into the parameters in the Social configuration area *should* work.
Have you tried that yet? If it doesn't work, let me know, we'll gladly investigate further.
Thanks,
Alex