That PDF is for proofreading and review, so I just used U.S. Letter size to make it easy to print on my home printer. For actual publication, I would use the page size for, say, a trade paperback.

For epubs, right now I'm using Pandoc: https://pandoc.org/epub.html. But, again, that's just now for the writing and proofing stages. For final output, I'll consider my options. (I may still go with Pandoc, but right now I'm using it because I was already using it for typesetting and so it was a low-friction choice.)