Let's talk aspect

At this point, if you want a truly responsive course that adjusts everything based on someone's device and browser width, develop the course in Rise. 

Unfortunately, Storyline (SL) slides can't responsively change their size. And I doubt that they will anytime in the near future.

For such fixed-aspect-ratio courses, my clients prefer 16:9, because it fits so well on laptops and horizontal phones, and looks good on tablets.  In other words: tough luck for those who don't like turning their phones sideways. 

If I were using SL for an audience that was almost all virtual, I'd still use 16:9. But I'd turn it "sideways" and make that height:width (not width:height).