13" is what you want... as the screen size rises from there, the units get much more expensive. besides, 13" is all you really need. I have both a 13 and a 17 and the 17 is just a behemoth and while mobile, I love the 13 much better.
there are so many in that range, what other specifics are you looking for?
do you need fast start times and schock resistence (SSD, but wont happen at 600$)?, do you need graphic performance? are you playing games?
remember, the higher the resolution the better the image but also the smaller the icons in that image (harder to read)
Amazon has a GREAT return policy, zero issues with new units.