I've recently got an iPhone 6 plus and I tried it out yesterday with Skydemon on a jolly into Denham. It worked very well and the screen size is adequate for flying. I usually use an iPad Mini with SD mounted on the main panel and in my opinion that's the perfect size. I'll probably keep SD on the iPhone as a backup in case my mini goes u/s.
On the iPhone 6 plus size front, I was initially unsure about how big it was but after a month of ownership I'm totally sold. It can be a little unwieldy one handed, but generally is excellent. It also has a much better battery than the iPhone 6 and optical image stabilisation too. It easily fits in my pocket too!
Sent from my iPhone 6 plus still with 45% battery after a full days usage...!