It's the 4.7", my shop didn't get the 5.5"
One handed operations main focus is navigating the entire screen with one thumb while securely holding phone, regardless of your activities. Nothing to do with texting and driving, the keyboard is still fully accessible with one thumb on the 4.7" model, that ain't gonan stop the dinosaurs from doing it.
Even with a full tactile keyboard in 2003 on my Treo 600, it was impossible to text and driving and I didn't even bother with it.
2011 to the present, b/c of Siri the car stereo is mostly off due to texting and driving with that feature. Even walking and texting is now used with Siri, dictation and even iOS 8 voice recording, still my other hand is free.
PS
Sliding, pushing and shifting a phone to reach the top of the screen is the #1 reason ppl drop & crack their screens.