While the answer to this might change depending on your chosen font, for simpler fonts such as Arial or Times New Roman, we typically recommend that you do not choose a font size under 10 pt.

Our default text boxes set body text at 16 pt and header text at 52 pt, but these settings can vary depending on your desired look. Keep in mind that more stylized fonts may appear differently and may not follow the recommended minimum. 

If you're ever unsure, you can always generate a PDF Proof of your book from the Manage Ladder screen to get a better idea of how the text will look on the page.