Larger fonts can be seen better when projecting to a group. Smaller fonts allow you to see more text.
Clicking on increases the font size; clicking on reduces the font size. Try it!
The font size control triangles appear only once on each page, near the top.
The font size you select will affect all pages. Returning to the main page (white background, no frames) resets the font to small size.
How to add font size control to an existing Chime tutorial.