Geometries in pixel size

Hey guys,
I'm looking for a way to draw geometeries like rectangle and circles in pixel size. Meaning I would place the geometry at a certain lat/long but set its size in pixels. For example to create a rectangle or a circle around a billboard. I know I can use an image of a rectangle with transparency in the middle but I would rather draw a geometry.
I could not find any reference to something like that in the geometries and apearance tutorial or anywhere else but mabye I missed something. Is this possible or can it be acheived in any way?

Billboard and Label have a computeScreenSpacePosition function that you could call each frame to position an HTML element over the Cesium canvas.

Patrick