In KMLer Pro i use Marker Style

1. Set need colors and size of markers in legend of current layer.

2. Set base Marker Scale and Label Scale in group Markers & Labels



3. Check "Use icon scaling" , Min scale, Max scale in



KMLer make proportional recalculation of symbols.
True Scale = Scale from dialogue "Marker style" * Scale from base dialogue "Export to KML".

You can use as markers also the simple brick or SketchUp model.