Reposting my earlier subthread comment here for easier access:
Hey there, I'm the developer of the new FlickType Keyboard, as well as the developer of the original Fleksy Keyboard. I second the sentiment of most comments here saying that screen size doesn't matter too much if you're going to be primarily using VoiceOver, although if you do have some functional vision then larger screens might offer some small additional benefit. I think you're really going to like the switch to iPhone once you get the hang of it, although I don't know much about Windows Phone accessibility.
With regards to on-screen typing with Flicktype in particular, screen size matters so little that you could even type on an Apple Watch with almost the same comfort and speed - we've built an internal prototype that worked surprisingly well.
If you're interested, you can join the FlickType beta and get the keyboard for free, by just sending an empty email
$1. Just don't tell anyone else though, because we really depend on the support from paying users through the
$1 ☺️
Disclaimer: I am not blind and do not regularly use VoiceOver myself, but I have a lot of experience with it.