Vectors defined in CZML are currently rendered using the PolylineCollection primitive. This was the easiest solution we had at the time support was added, but makes it impossible to implement the feature you’re describing. At some point in the future I would like to re-write the vector visualization to be more flexible and will definitely keep this request in mind when doing so.
One thought I had was to render vectors in 3D using Geometry & Appearances; at that point we may be able to do something with the primitives model matrix to easily scale the size based on camera distance. But this is just me brainstorming, there might be better ways to do it.
I know other people have asked for a similar feature with models (drawing them at constant size until they are at their “real size” based on camera view).