In almost all reading devices, users can change the font size. In the Kindle, the font can be changed from the default size of 3 down to the smallest size of 1 and the largest size of 8. (See our the screenshots of an example ePUB in this micro-article FAQ: Text reflows--or wraps.
In many ereading devices, the human reader can even change the font style. This will also affect how the book looks, not only in the font. This will change the spacing between letters and words, changing your book yet again. For this example, we thank Connie Pontious, author of the charming children's book, "Sharon and Eleanor," for allowing us to use her lovely work for our example. ("Sharon and Eleanor" is available on Kindle by clicking here.) These screenshots were all taken from an actual Nook device. Top row left: The book with "Ascender Sans" font chosen, default size; Top row, right: The book with "Georgia" font chosen, default size. Second row, left: the title with Georgia font, smallest possible font size; Second row, right; the same page, also with Georgia, using one of the largest font sizes available.
What's important to remember, when you view the above images (you can click any of them to launch them in a lightbox), is this:
...this is exactly the same book, on the same device. The only thing that's changed is that the person reading it has chosen a different default font to read it with, or a different font size. That's why making eBooks is so tricky--the person who formats them has to be able to ensure that no matter what your reader does, the book will still work, function, and read properly, despite the reader's choices.