Well, my baby was 6 lb 5 oz at birth, and he was 10 lb by about the end of his 5th week (granted, that's a pretty fast growth rate, and not all babies gain that quickly). So, if you go with the 11-14 lb package listed here, you may be waiting more than a few weeks to start with the cloth diapers. Of course, you could have a 9-10 lb baby, then you really would be only waiting a few weeks until he/she fit the small covers. I know that planning is kind of a pain when you don't know how big the kid will be at birth. I would base the decision about which package to buy on whether or not you would be annoyed if you bough the XS stuff and were only able to use them for a couple of weeks (i.e., would you feel you'd wasted too much money).
With the covers - the length of time you can use them really seems to depend just as much on the shape of your baby (long and thin, short and thick, chubby thighs, etc.) as the weight, so there's no way of really knowing ahead of time.
Sorry, that's probably not the answer you were looking for.
