I’m pretty sure that setting is only when using text instead of a slash:
You can still change the slash though, it’s just a bit more work. Go to Engraving Options/Chord Symbols/Project Default Appearances/Edit, enter any slash chord, and hit +. Bb/C is fine but we really only need a way to edit the slash so it doesn’t actually matter. Now click the slash. Any edits made here are just for that particular chord, but if you hit the Edit Component pencil underneath, you can now make a global edit to the slash. In the next editing window scale and position the slash however you want and it should affect all slashes in your project. If you save your settings in a default file that you normally start with then you won’t have to deal with changing it again.