I was worried about this as well when I decided to get a reader. I've used my 6" reader for a couple of weeks now and I don't think I'll upgrade to a 9" screen. There's not that much difference between the 6" reader screen and a page of a normal-sized paperback (depends on the font size of course, I use the small font size). You don't notice the page turns, just as you don't really notice turning the page of a paper book.
And then there's also the portability to consider: I wouldn't want to carry a 9" screen around everyday. I might as wel just take a book with me then ...
I guess the smaller the better (at least where ereader are concerned
)