There are two options to fix this:
1. Inspecting the widget on your page, the .fb_iframe_widget class is getting a left of 7%. That is coming from what looks like a custom module or style block on your page. If you shift this to 0, the entire comments box will fit.
2. There is also a width parameter when the Comments widget is selected. You can enter a pixel amount and, I believe, 100% if you want to fill the content area. Just take into account the 7% left in whatever value you put.
Hope this helps
-Melissa