Best slide size for responsive course using Storyline 360

Hi all,

I'm guessing from this thread that it's not actually possible to create a neat looking one-size-fits all-devices course in Storyline because of the aspect ratio difference between tablet (4:3) and mobile phone (16:9). i.e. the screen will not neatly fit to the size of the target device - there will always be a 'letterbox' effect.

For example if I go with a 16:9 ratio, on ipad it displays like this:

However, the Storyline 360 features page on the Articulate website seems to indicate that you can actually do this without any 'letterbox' effect. (https://articulate.com/360/storyline)

So I'm just wondering is it possible to achieve the result shown above? i.e. one single course built to fit neatly on all devices without 'letterbox' effect? If so what are the settings that are used in Storyline?