In the process of searching for patterns for preemies, I found this site. It gives information regarding sizing for hats for babies from 1-6lbs. There is also a pattern included for a baby that is 5-6lbs. I am sure you could adjust this for smaller babies by changing hook size and a few less rows.
This is a phenomenal resource! I have made loads of hats, but was never really sure what size baby they would fit. Hopefully this will help me make a good mix of hats for all different size munchkins.