I have designed an ebook using very specific formatting and graphics that need to be maintained. As such, I am converting the individual pages into image files, and then publishing the complete book as a pdf for uploading as an ebook. What point size should the text be on the page images to not be too small for reading on mobile phones, tablets, and e-readers?
I would say a font size of 12 for the body text and 14-18 for the title.
You may also check some of the suggestions for the font types at https://creativemarket.com/blog/best-fonts-for-ebooks
The same website also has some suggestions about the font sizes, which seem to be matching what I am suggesting.