There are no fixed pages, it is reflowing. If you need to make changes, search for the page you need to edit (search for the text) and make your changes. The book has probably a specific xhtml document for each chapter.
Font sizes are best described in the unit em or percentages and can be set in a stylesheet. Define a style for your paragraph with the font size in it and make your paragraph use that style.
An example:
In your stylesheet:
Code:
.bigfont {
font-size: 150%
}
In your xhtml:
Code:
<p class="bigfont">Your text here.</p>