Reply To: What size to make the original InDesign document

Keep in mind that the page size you start with in InDesign has no bearing on the final product, because ePubs are flowing documents that can be sized up or down by the eBook reading device. So set the book up as you would a print book, using whatever page size works best for your project. But since you are working on a picture book, you will want to be sure that your images are sized to fit most eReader screens:

https://en.wikipedia.org/wiki/Comparison_of_e-book_readers

Looking at the list, you can see that most eReader screens are at least 600 pixels wide, so I would aim for keeping my photos and illustrations to that width. Or, you can create a “photo” style that will set the width of your photos to 100% width, so it will automatically adjust to the width of the particular device.