So, where exactly is the line?

I didn't much more than skim the Surface review, what's the point?

AT and Ars will offer better technical reviews; the UX for someone who doesn't know or care about the Microsoft ecosystem isn't useful information to me; and the comment section is going to be a cesspool, a plague on all of our houses.

But I did notice one interesting thing:

iPad Mini: screen width 4.73", 768p verdict? "The display on the mini looks incredibly sharp"

Surface: screen width 5.2", 768p verdict? "At the size of the Surface screen, 1366 x 768 resolution leaves much to be desired"

I have a hard time believing that less than half an inch is the difference between "leaves much to be desired" and "incredibly sharp".

So. My question is, what is the magic size where ppi is just good?