Is there a way to set a scale/minimum size to an ellipsoid the same way that minimumpixelsize (and maximumscale) does with a model?

Question

I want to use a czml to draw some spheres floating around an area of the globe, and I want them to keep the same size regarless of zoom (within limits). This is trivial with a gltf model, but I cannot find a way to do it with an ellipsoid.