First, a bit of background info: I've been an iPhone user for the past 4 years (less than most of you here - I'm sure). My first was the iPhone 4, then 5, then 6. I've had the 5 for two years and do not recall experiencing any eye strain - even during the initial adjustment period from the 3.5" iPhone 4 screen to the 4" iPhone 5 screen.
Now, I do understand that this is a thread for the iPhone 6 plus which boasts an additional 1.5" inches of screen, but let me describe how I've been feeling since I switched to the iPhone 6. I've had the 6 for about 2.5 months now, and stare at the screen for approx. 4 hours per day combined. I noticed significant changes in my ability to focus on objects at a distance - *note that this goes away after a good nights rest but returns after approx 20 mins of phone usage*. I tried not using my phone for a day and my eyes were not as strained and my distance focus was better. I wouldn't say I've gotten migraines, but significant eye strain - definitely.
I think this has something to do with the convergence angle of the eyes on a bigger screen rather than a hardware problem. My suggestion is to hold the screen further away from the face; even though walking around with outstretched arms is not exactly practical. The curved edges of the phone make it more difficult for our eyes to discern where the screen actually ends, thus straining the peripheral vision - holding the phone further away from the face will limit the 'infinity edge' effect.
Meg, to be honest, your comments are the reason I decided to post on this forum. I've been in the US for 7 years, and spent the 10 years prior in Europe. In Europe, politicians are not 'guided' through the decision-making process by powerful lobbies. There are very few such lobbies capable of suppressing true scientific research, which is what decisions in Europe are based on. Radiation exposure is a legitimate concern - of course the regulatory agencies have imposed SAR requirements - which, by the way, differ from country to country. The effects of microwave radiation are quite real, perhaps not in the quantities emitted during normal phone use, but in general - yes, they are real. I need not go into the details, but with that in mind and given that "normal phone use" these days is practically "all day, everyday", it is perfectly reasonable to assume some people will experience adverse effects - Please respect their opinions.
It's worth noting that radiation has an extremely long half-life and will thus accumulate in our bodies during our lifetime - we're young now, but we're the first generation to actively use a cell phone... who knows what health problems this exposure would have caused by the time we're 65. I'm just saying ...