Storyline size for LMS

The screen display dimensions are pretty much determined by your LMS. And most LMS players that I know of permit the end user to toggle full-screen coverage if desired.

For example: We are using SuccessFactors. By default, the player for a course takes up probably 70% of the visible screen; there's a wide margin (literally) around three sides, and the top of the screen displays the SuccessFactors information bar (just in case people forget that they are in SuccessFactors...). But toggling the course player display to full screen expands the course to fill the available screen, and that size depends entirely on the individual system that plays the course.

So I would suggest that you set your default slide dimensions to whatever you prefer (ours are 16x9 to correspond to the corporate PPT template...) and then include the screen display toggle as a user-controlled setting in the player.