It's currently possible the scale the HP bar, but not the cast bar. Which results in different sized components, with no way to edit.

Castbars are a bit weird currently when it comes to scaling and positioning and basically just breaks for certain themes. Which is why this doesn't also scale the castbar. I think I just need to redo how they are set up but it's pretty low on the list right now.

However, I could add in another slider underneath it that does the same thing, as width mostly works but not always. So I can't guarantee it will work for the theme you are using but at least it would be something for now.

Me, specifically, am using the Blizzard theme.
Just a slider to adjust width would already be dope.
I've got scale on like 60% and the castbar is just giant in comparison.

Thanks for taking this into consideration.