Smart Map - setting an icon size
As of Smart Map v2.4.2, it is now possible to build more elaborate markers using a nested syntax for the markerOptions value. Try something like this...
{% set options = {
  zoom: 6,
  markerOptions: {
    icon: {
      url: 'https://www.mysite.com/images/map/marker.png',
      scaledSize: 'new google.maps.Size(20,20)'
    }
  }
} %}