Page size does not matter
I have read around this subject a lot. I have concluded it does not really matter because people might read you e-books on all sorts of different e- readers, although you cannot just make a single e- book and assume they will work on all readers.
Forget page numbers
It looks to me like A4 and A5 are going to be fine. More importantly, while you may not need page numbers (convention says you do not,) you may want to make it so a page appears as just that.
This means there are some issues to think about in terms of what you put on each page.
Set the margins
For years I did not realise you can simply type the size of the page and the margins you want into the word processer formatting option and spent hours scrolling numbers backwards and forwards. About an inch all round seems about right.
Remember that you need to think in terms of a web page and not a physical book so you do not need to leave extra in for the spine. This is one reason why you cannot simply upload your book written as a paper or hardback as an E Book. You will have left an extra amount on one side for the spine.
Limit the number of words on a page
There is apparently, an industry standard of 250 words per page in an e-book.
It is more important to know how to make the headers.
Use H1 for your title.
Use H2 for your chapter headings.
If you don’t you might get a PDF past somewhere like Lulu.com, but you won’t get your E Book into Barnes and Noble and Amazon as they will throw it right back at you telling you it is not correctly formatted.
Keeping these things in mind, you can begin to write.