What units can you use?

You can use any unit that can be used in CSS: em, px, %, etc. But I suggest using px because some themes need to adjust the width and the height to make the image smaller than the panel and if you use em or %, it may fail and you can get a strange behaviour.

In any case, you can try with em and % and if it doesn’t fail, you can use them. If the carousel is broken when you use that unit, just use px of modify the theme CSS.