Best slide size for responsive course using Storyline 360

Here's a thought...  It seems that whatever ratio you choose, on one device or another there will be letterboxing.  4:3 will end up letterboxing on the sides on those devices it doesn't suit, and 16:9 will typically letterbox top and bottom.  Obviously, if you have a specific device target (as in your clients use a particular device), then choosing your ratio is a matter of pleasing your client and relevant learners.  That may mean neither 4:3 nor 16:9.  If however, those restrictions aren't the case, consider this.  

In Western culture we are accustomed to reading left to right.  In our courses, we are not only using text, but we are also relying on eye engagement as it responds to visual communication.  Between 4:3 and 16:9, a 16:9 ratio leaves maximum room for left-to-right design space (and eye engagement) per single sweep.  Again, culturally, it has become the standard expectation for those who engage in a visual way through any device.  So, if I had to choose between 4:3 and 16:9, I would go for the option which leaves room for maximum engagement before interruption.  

I recently designed something in 4:3, but I am now going back and resetting it all to 16:9 for the above-mentioned reasons.  I hope that helps someone.