In the Googlemaps plugin you can scale the icons based on the definition in a kml file with the geoxml renderer.
For this option you still need to use Google Maps API version 2 so use version 2.18 of the plugin and set the google API version to 2.x.

Example for a kml file:

<IconStyle>
<scale>2</scale>
<Icon>
<href>http://maps.google.com/mapfiles/kml/pal4/icon61.png</href>
</Icon>
</IconStyle>

{mosmap kml='http://www.fourdirectionsmaine.org/map/tribes.kml'|kmlrenderer='geoxml'|kmlscale='1'}