I have found that the selection of a medallion is more influenced by the ceiling and the room and less about the light fixture itself. A big ceiling, one that is high as well as wide and long looks best with a big bold medallion. Think of it as a part of the ceiling and not a part of the chandelier.

Great Comments. In this case, the ceilings are not high (9') and the chandelier is rather
delicate. In this case, I think it will be a judgment call. Will gather some examples and start there. Thank you.

I say larger then the chandelier .

Many thanks ... I think like you. Why bother if it does not at least make a statement. It can be simple but big.