I love my 5" 350, but it's sadly neglected in favour of its larger cousins, because apparently the bigger the better.
I don't think a 5" is too small, and I find the inch of difference makes the device
very portable. I think it's best for somebody with decent eyesight (or conversely, somebody who wears glasses all the time and therefore text size doesn't make any difference to
me them). I know on the 350s, you probably don't want to read at large text size or above regularly. You'd be turning the pages a lot if that were the case, and I'm pretty sure this goes for any other 5".
You're best comparing the two sizes in person, if you can.