Update listing.html

Author: MichaelCurrin

_layouts/listing.html

@@ -7,7 +7,7 @@
     <header class="post-header">
         <h1 class="post-title">
             {% if page.icon %}
-                {% include icon.html name=page.icon %}
+                {% include icon.html name=page.icon _size='medium' %}
             {% endif %}
             <span>{{ page.title | escape }}</span>
         </h1>