If you have (and you definitely should) create a registry, you should include the diaper size that you'd like. In the event that you get sizes that won't work for your newborn, you should be able to exchange them for what you actually need as long as they are unopened. Target and Babies R Us will let you exchange without receipts or put the amount on a giftcard for you. Good luck!
This. Also, WalMart will take unopened diapers back.
Although I see your point about wanting larger sizes (especially since you'll come home with a boat load of NB's from the hospital) hopefully most people will give you size 1-2. My DS was 9.7 pounds and he wore newborns for 1-2 weeks. You go through a lot of diapers the first few weeks or so. lol