What dimensions do I use in Word for a document that is going to be viewed on an iPhone

I must be asking a really dumb question because I cannot find an answer on the Internet. I cannot even find where there has been a discussion of anything remotely associated with my question. And lest you, in kindness, remind me that Carl Sagan proclaimed that "There is no such thing as a dumb question," I must tell you right off the bat that Carl and I greatly disagree. Not only is there a such thing as a dumb question, I think I am asking one right now!

Here is the issue: I am creating a document in Word to be viewed on my son's 15 Pro Max iPhone. My question is, what height and width should I use in Page Layout for a document that is to be viewed on a mobile phone?

See. I feel even more dumb just having written that question! Still, I do need an answer pretty quickly.👩🏽‍💻🙄🫣