Just get newborn stuff and if she's too little the hospital will have pjs that she will be able to wear. But diapers buy newborns and if she's gonna be in the hospital they will supply premie diapers also. You can buy some but the hospital will use theirs.
I would get maybe an outfit from each size just to be on the safe side. I have newborns, 0-3 months, stritcly 3 months, and even 3-6 and 6-9 if they don't fit now when she comes they will later.