What's the best page size for easy readability?

I make flipbooks in both desktop and mobile versions. When the book is viewed on computers, it looks large but much smaller on mobile devices. So what page size is the best for crystal clear viewing?

The flipbooks created by our page flip software are responsive, which means the flipbooks will scale automatically to fit the screen and browser window. So it doesn't have an effect on readability whether the page size (of source file) is large or small. You can decide that according to your needs and a standard size (A4) is recommended.

What really matters is the design of the page layout and content. It is advised that the display content should cover most part of the page without affecting the aesthetics of your artwork.

And one more thing you should keep in mind is that the display content should never be too dense. People would not like to read a page with thousands of lines of tiny words. Make your content concise, readable, clear and attractive.