I use SD on an iPhone 6 plus and I feel it's an excellent screen size. I also use SD on an iPad Mini, which one I use depends on the length of flight, but the other is usually kept as backup. As others have said, secure mounting is essential as you'll need to be able to press the screen without it falling off. If I had to pick one device for SD I still think it would be the Mini, providing you've got enough room, however the iPhone is a very close second. I tried SD on my older iPhone 5 and found the screen too small.