What's the maximum necessary size in pixels for a video for iPhone?
The currently biggest iPhone is the iPhone 7 plus. It has a 5.5-inch (diagonal) LED-backlit widescreen.


In landscape mode it has a 1920-by-1080-pixel resolution at 401 ppi.
In portrait mode it has a 1080-by-1920-pixel resolution at 401 ppi.


Which is the equivalent of a full HD movie.