My test is - can I touch all four corners of the display with my thumb. If I can, it's fine for one handed use. My smartphones have had 3.8 inch (LG Optimus 7), 4.5 inch (Nokia Lumia 920), 4.2 inch (BlackBerry Z10), 4.5 inch (Nokia Lumia 1020) and 4.5 inch (Jolla) displays.

I loved the LG's display at the time, but when I tested it again recently, typing felt too cramped, and the browser was frustrating on full desktop sites. The 920 was absolutely brilliant in the hand, 4.5 inch felt fine, extra screen space was nice to have (and the display itself was lush). The BlackBerry Z10 was probably the sweet spot at 4.2 inch, and I felt a little uncomfortable when I went back up to the 4.5 inch Lumia 1020. That may just have been because of the huge camera bump though, because the Jolla feels great in the hand...

My point? Screen size doesn't make a huge deal of difference - it's the other dimensions that count.