I hope that the area with the red frame could show words too.
You can adjust text and page size under the settings button (gear icon) on the top right at your screenshot.
But when I adjusted the font, only the font became smaller, it didn’t show more content. As you can see, the lower half of the page is empty.
Try changing the page size under the settings, not font size. Also, specific lessons may have formatting issues, I’ll look into fixing the one you posted above.
from my end I would add that these formatting issues described by zhouzhou are present in all my lessons. Insufficient text on my pc, but no issues on my ipad. Font or page size changes make no difference.