Looking at the examples for stacked icons, you can do this by modifying the font-size on the span:
Just add a rule like:
.fa-sm {
font-size:0.63em;
}
With Markup:
<span class="fa-stack fa-sm">
<i class="fa fa-exclamation fa-stack-1x"></i>
<i class="fa fa-circle-o fa-stack-2x text-danger"></i>
</span>Github Issue
Fiddle: http://jsfiddle.net/epxtY/