I use iphone 5s and ipnone se, but the tips and warnings fill the left part of the screen so I can't see where I am going.
I can't just dismiss them while my fingers are busy with the sticks.
What can I do about it? The screen is just 4"

I have a iphone 6s, and i close the warnings after reading them.  only takes a fraction of a second.  Plus you need to move your fingers off the joysticks anyways to read the warnings.

That is exactly my concern when I think about using my iPhone SE.
Currently I have not made a purchase (drone), but I do own a SE, so even though it is 'compatible', I am worried it is not enough. (size)

I use the iPhone SE with my Spark. I don't have any problem with the screen.  I have an iPad Mini 2 that I can use, but I prefer the SE.  It makes a very compact package with the remote.

I found a free 3rd part iphone app which lets me see the whole fpv view.
It does not have quickshots (yet).
It’s called vr pro for spark/mavic/phantom and have worked well for me. It even has a fullscreen mode.

The only limitation is that the goggle option (split display) only works for 60 seconds.
Not enough time to try it out. Maybe I will pay anyway. It’s very cheap.
This is the US version: https://itunes.apple.com/us/app/vr-pro-for-spark-mavic-phantom/id1255295101?mt=8