I have to disagree on this. 7 inches are so much better when you want to read on the crowded NYC subway. I already tried with an iPad and it's a whole lot more inconvenient.
It might be because my eyes are still young, but I prefer smaller screens with a high pixel density to larger screens with a lower pixel density.
It is because your eyes are still young.
7" is definitely a good size for tossing in a courier bag/coat pocket though.