In the Googlemaps plugin (version => 2.12o) 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='http://www.fourdirectionsmaine.org/map/tribes.kml'|kmlrenderer='geoxml'|
icon='http://tech.reumer.net/images/demo.png'|iconwidth='64'|iconheight='64'|
iconanchorx='32'|iconanchory='64'|iconinfoanchorx='32'|iconinfoanchory='1'}