Font Awesome icon size not increasing

For font awesome icons sizing please check this link. http://fontawesome.io/examples/

To increase icon sizes relative to their container, use the fa-lg (33% increase), fa-2x, fa-3x, fa-4x, or fa-5x classes.

<i class="fa fa-camera-retro fa-lg"></i> fa-lg
<i class="fa fa-camera-retro fa-2x"></i> fa-2x
<i class="fa fa-camera-retro fa-3x"></i> fa-3x
<i class="fa fa-camera-retro fa-4x"></i> fa-4x
<i class="fa fa-camera-retro fa-5x"></i> fa-5x

To increase icon sizes relative to their container, use the fa-lg (33% increase), fa-2x, fa-3x, fa-4x, or fa-5x classes.

            <ul class="stay-connected-inner list-inline">
              <li><a href="#"><i class="fa fa-twitter fa-3x" aria-hidden="true"></i></a></li>
              <li><a href="#"><i class="fa fa-linkedin fa-3x" aria-hidden="true"></i></a></li>
              <li><a href="#"><i class="fa fa-medium fa-3x" aria-hidden="true"></i></a></li>
              <li><a href="#"><i class="fa fa-linkedin fa-3x" aria-hidden="true"></i></a></li>
            </ul>

font-awesome icons are texts. So use font-size to increase the size of font-awesome icons.

For eg.,

i {
    font-size: 18px;
}