In my experience phone screen size didnt matter much. I use an iphone 8 or pixel 4a (significantly larger screen compared to the iphone) when flying, the bigger pixel screen didnt really make a difference because fly app didnt utilize the entire screen for the video display, top end (or left side since its in landscape) of the screen is just black. So I end up with essentially the same display size as the smaller iphone.
What I would suggest though is to use a dedicated phone if possible, this way you can uninstall / disable all the other apps that you dont need so that fly app is able to use as much of the processor as it needs.