Just picked up my 12P today after a year with the 11PM. Definitely still getting used to it. Making some typos but fewer as the day wears on. I feel the size and weight difference in my hand pretty easily, especially in my left pinky which I place under the phone (or felt I had to with the PM...maybe with the P I will lose this habit).
As for the screen size, still getting used to it. I use all sorts of accessibility stuff...bold text, larger text, button shapes, reduce transparency. I find all that stuff goes a long way toward customizing and helping with the adjustment. To be clear I also used this stuff on the PM.