I've read that being overweight can contribute to having a larger baby, but of course genetics (and just plain luck) also come into play. And I'm not saying all overweight people will have large babies, but it's just something I read could contribute.
My mom had 7 kids; the first was about 6 1/2 lbs, and after that we each went up about half a lb to a lb (with some exceptions) until the last ones were over 10 lbs. I tend to think this is because my mom was not overweight when she was younger, but became increasingly overweight with age and each child (took her 18 years to have all of them).
I'm not tall or overweight (yet, anyway), and my husband is actually on the short side, so I'm betting on an average-sized baby that will fit NB clothes. But I'm still not buying much NB in advance right now. I mainly want a few little undershirts and sleepers for photos, so that everything is not baggy in photos (I want to remember what those little arms and legs look liked!) but I'll get mostly 0-3 month size for now.