The different sizes of socks for the babies can be created by using larger crochet hooks and heavier weight yarn. I have found that using Opal sock yarn and an F hook, gave me a preemie/newborn size sock. If the preemie was smaller, I would use a smaller crochet hook and perhaps a lace weight yarn.

In the same manner, to enlarge a sock, I would either use a larger hook, heavier weight yarn or use more stitches to create a larger circimference sock.

Preemie Size =F hook and fingering yarn
Size F or G hook for Fingering
Newborn and larger sizes - use G or H hook and Sport, DK or Worsted weight yarn.