It's more likely that the your screen size will be influenced the most by the physical application.
<> Decide where in your auto your screen will likely be placed.
<> Get the largest screen that will fit.
+ This will get you the best viewing experience
∙ You'll spend more time looking at your screen than fingering it.
∙ Button size is mostly a function of resolution and the app displaying buttons
& Those that are 'touch screen' friendly have large enough buttons for the masses. --unless you have huge oak stumps for fingers --do you?
∙ You may feel a bit clunky at first w/ your screen and buttons. They're new.. you may be driving at the time. but in time, You'll get the feel for 'em.