SPPB does not affect the width. As you can see in the screenshot SPPB content block fills the full width of the the bootstrap container. All that is below the highlighted line of code refers to the FB module, but not to SPPB.
Here is a piece of code can help to adjust the width of the box that contains the widget.
.fb_iframe_widget > span {
width: 800px !important;
}
But part of the code which adjusts the width of the widget is outside of your site. And you just can not change it through code.
Ask FB module developer.