There's really no great way to tell how big your baby will be. Even estimates made by doctors from ultrasounds can and have been drastically wrong (for example, my friend who agreed to early induction for a "very large" baby- supposedly in the 9/10lb range- and ended up with a 4lb 10oz nicu baby). My mom was 4lbs, my nana was 5, I was barely 6 lbs even over a week late, but my husband was 11 lbs 8oz! I had no idea what to pack! The best advice I got was to pick out one outfit from each range of newborn, 0-3 months, 3 months, and 0-6 months to pack in the hospital bag for going home. That way you're pretty much covered all around. Preemie clothes can be picked up afterwards if needed, or you can include an outfit from that range too.