In the Googlemaps plugin you can define the base icon for the kml rendering with kml renderer Geoxml. Now you can control the size and all other parameters, see: custom icon.

{mosmap kml='/images/tribes.kml'| kmlrenderer='geoxml'|
icon='http://tech.reumer.net /images/demo.png'| iconwidth='64'| iconheight='64'|
iconanchorx='32'| iconanchory='64'| iconinfoanchorx='32'| iconinfoanchory='1'}

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