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If you changed the font size of books, then follow the steps below to change it.
Open a book (you must open a book or text to make this work). Then will in a page, tap on the top of the page and you will see a Aa button or press Aa button you see on toolbar/keyboard. Tap/select it and it will allow you to change the font size.
If this is Kindle Paperwhite, then open a book, then pinch in or out with two fingers and it will increase or decrease fonts. Or do the following;
Slide your thumb and pointer finger together in a pinching motion on the screen. To make it bigger, quickly slide your thumb and pointer finger away from each other on the screen and the text will automatically grow by one degree.
Please let me know if that didn't work, so I will continue to help you