If you go to the link you gave you will find the live DEMO link.
Open the demo page and view page source.
You will see this comment in the html...
<!-- Custom JS that sets: Zoom, Pointer and Map Type; Change the class name of the <div> where you want the map: current class name is "map-location" -->
They have set the map to be shown in...
<div id="map-location" style="width:100%; height:300px;"></div>
Looks like a little conflict in the html comment since it's an ID and not a class.
The script targets the div with
var map = new google.maps.Map(document.getElementById("map-location"),
Anyway, you can see the dimensions are set on that div, which could be set in the CSS file rather than inline styles.
Keeping in tune with your thread title "Responsive Maps" I would stay away from any rigid fixed dimensions.
You can get some flexibility by using min/max sizes along with fluid dimensions.