For what it's worth, I'm also a firm believer of "it should be useable with one hand without akwardness". 4.5" would be my perfect size; just slightly smaller than the current small offerings on the market. I currently have an iphone X and really dislike the form factor - it's just too large, and the rounded corners lend themselves to slipping out of the hand. The square edge design really was the easiest the grip - I hope it makes a return on the next iphone pro. The ipad pro has gone back to the square edge design already, so there's some small hope in this regard.