page size in indesign in order to export to epub digital editions and fill all the page

Can't really improve on Bob's answer, but I can clarify that you're sort of chasing a rainbow, here.

To fill an EPUB reader screen, the page proportions have to exactly match between source and that screen. While you might be able to succeed at this for one reader, or some subset of readers, the full spectrum of them not only vary but vary further according to how the user has them set up and configured. It's a variable over which you have no control, so staying with about a 1.3:1 to 1.5:1 ratio (letter to 6x9, with A4 in the middle) is the best you can do.

And that applies to fixed-page layout or FXL, which should no longer be used except for a very small subset of "picture page" books like children's and graphic novels. There, a fixed page size will be subject to the screen proportions and size issues that you can't do much about.

And it's all irrelevant for reflowable EPUB, which simply fills each screen from margin to margin, top to bottom. Reader margins are reader-controlled (not document-controlled) and often hard-coded, if not fixed then a limited number of choices like "narrow-normal-wide." But it's the reader and the user calling that, not your doc.

So: use reflowable, and use any convenient standard page proportion for source and export, and allow the end user/reader combination to define display proportion, spacing, margins etc. — mostly because you have little choice in the matter anyway.

Page size is irrelevant in reflowable EPUB. Fixed layout EPUB is a garbage format. ADE should not be used under any circumstance. It's never been good at either reflowable or fixed layout and hasn't been updated in forever.

On Mac use the Books app and on Windows Thorium Reader.

Hello

Sorry I don't understand the query - what is the issue? What is the results you expect? 

It's a bit unclear for me.