Each presentation maintains a constant width/height ratio, even if you resize or flip your screen, so that it nicely fills the screen, similar to a video. The default ratio of 16:9 (also know as ""widescreen") is recommended, since most modern TVs and phones have this (or close to this) display ratio.
To change the ratio: Click on "Choose your media" to expand that section; then choose a new presentation ratio from the dropdown. Observe the size of each slide changing, as you do so.
Note: If the presentation ratio does not match exactly the viewer's screen ratio, there will be black bands on the sides or on the top+bottom of the presentation. This is unnoticeable, as long as these bands are small.
We also recommend choosing images that are horizontally oriented, so that most/all of the image will be visible on the screen. Read more here.
There are few cases where you may want a ratio that is different from the default "16:9" :