Google Maps (and GGeoXML in the Google Maps API) - always renders
icons as 32x32 pixels - no exceptions, the KML cant change that.
It's mainly down to a technical limitation, as the javascript doesnt
know the icon size before rendering - ok there are work arounds but
probably non-trivial.
If you using the API, then there are alternatives to GGeoXML which can
cope with different size icons - the source is available so can edit
it.