Hi Ralph,
Have you tried doing so in your site’s CSS? What was the result?
The problem is that a fixed width is injected into the style attribute of the html so can’t be overridden with the themes css.
The width is set in the widget itself in px only. Maybe a drop down could change it from px to % as this is a real problem for a responsive layout
I suggest you open a github issue for this here, then:
https://github.com/Automattic/jetpack/issues/new
If you’d rather I do it myself, let me know. You could even submit a Pull Request yourself, if you’d like:
http://jetpack.me/contribute/
As you saw there, it was closed as invalid because Facebook actually requires the width be in px; sorry!
Should I do another one for the twitter widget then? That at least could be dynamic width?