The standard for 121/135 is that the electronic device can show you an approach chart and you can read all of the chart w/o having to zoom. The old standard was it had to show the chart "full size", but the FAA has changed that. So you can have it displayed and keep hands on the airplane controls. The mini will do this for most people, but a phone does not. Far-sighted and older folks may still benefit from a larger one. Phone could be a backup to the ipad or other device. If the airplane cockpit is big enough, a standard size ipad may be the way to go, but usually cockpits are pretty constrained. While there's no requirement to follow this, it would probably be a good idea for general aviation, the entire idea is that the ipad be usable in flight and not create more problems than it solves.