I often get asked the question about choosing the right slide size in Articulate Storyline. The Slide size or as it's known in Storyline as "Story Size" in not one of those things that you really come across casually in Storyline. It's a bit out of the way.
When creating a new project in Storyline, you are not presented with a dialogue box to input slide size. Therefore, Storyline simply goes ahead and selects a size default for you which happens to be 720 x 540 pixels.
The more important value in project slide dimensions is really the aspect ratio which happens to be 4:3. (Width/Height for 720/540). This default aspect ratio is the most commonly used in computer monitors, TV sets, and most tablets. The other aspect ratio is 16:9 which is suitable for widescreen devices and should be used only if you know that your audience is using 16:9 displays.
Accessing the area in Storyline where you can modify slide size and aspect ratio should you decide to do so, is under the "Design" tab in either the story view or slide view.
The default slide size of 720 x 540px is a throwback to the NTSC video standard. One reason you may want to modify this default is if you need more canvas (real estate) in your slide. Perhaps your design and content require more space or just need to spread out.
Another factor that may need consideration when selecting slide size is the "Player Size" used when publishing your project. The default player setting is "Lock player at optimal size". This setting forces the output to maintain the scaling at exactly 100% of pixel dimensions (no scaling). Modifying the "Player size" to "Scale player to fill browser window" will scale the project regardless of the set width and height.