I've just made an experiment - and it works. There can be a choice of not 8 but more font sizes. I've made 11 options:
For this I modified the array
\framework-api\com\amazon\ebook\framework\resources\UIResourc es_en.java
like this:
"fontmenu.default.font.size.list", new int[] {
21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
}