Which slide size do you recommend for viewing courses on cell phone, vertical?

Hi there.

My client tells me that his employees use primarily their cell phones to navigate online courses built in Storyline, so it´s important that the elements in the screen (graphics, text, buttons) look good and the interactions feel comfortable in vertical position. So, what slide size would you suggest to achieve a good user experience, one that works acceptably for the many screen sizes of different mobile phones?

I tested a 720 x 1280 size for my Samsung J5 phone, and the result looks quite good to me, though not perfect (there are spare zones surrounding the screen when I look at it in Review 360).

I would ask if you have dealt with the same issue and what solutions you´ve found. Thanks!