Still using a Xperia Z1, its not working out for the one-thumb to rule the phone.
I actually hate it ALOT.
Next phone I will be picking something smaller even if the performance goes down.
(But I do like taking pictures with my phone)
There is no sweetspot in inches, you have small or big hands so get a phone that fits in your hand.
(looking at the Z5Compact from Sony are looking mighty interresting)