Images are mostly self explanatory. The first two columns show the default Kindle fonts displaying a page of text. The third column shows the sans-serif font displaying a page of text.
Each row represents a different size selection. Row 1 is size 1, row 2 is size 2 and row 3 is size 3.
Note that the sans-serif font is actually smaller than the serif font and shows considerably more text per page. This isn't a given. It is just that the font I used is a smaller font.
You can click on each image to see a much larger version with more detail.