I'm aiming for enough newborn stuff to get me through about two weeks, assuming frequent enough washes in that time but without having to wash everything frantically around the clock either, which I imagine is the last thing new parents need.
I was thinking if you take an average of 2-3 onesies/babygrows a day (allowing for having a baby prone to poo-splosions and big milky sick ups), then consider how frequently you think you will do a load of baby laundry, you can work out how many onesies and vests you'll need for approximately two weeks to ensure you always have some clean and dry and ready to go if needed.
(I'm going with two weeks as it seems to be the average time babies in my family have lasted in newborn, and seems to agree with what I have read here also).
I don't want to buy loads and loads in newborn size as I don't want to waste clothes with only one or two wears or none at all - if possible! However, as you said, don't want her totally swamped in 0-3 if she's smaller.
We are aiming to also have about 5 onesies and vests each in 0-3 (just enough for a few days) before she gets here in case she happens to be bigger - that will be enough to keep us going at the beginning anyway, till a shop can be done! Will bring one 0-3 onesie and one 0-3 vest in my hospital bag in case she they are needed and then if so, DH can bring the newborn stuff home and the rest of the 0-3 stuff in and we will have enough to work with in those early days.
Not buying any preemie/tiny baby stuff as most do not need it - I know of course that many do, but most do not and I think in the interest of not wasting money/clothes, it is safe to assume newborn is the standard size (or it would not be called that - though I agree with annoying discrepancies between brands!). If she is really little, she can go in an overly-large newborn onesie to begin and my husband or family can nip to the shops for some smaller stuff. Our hospital is right in the city centre anyway, and the parking is in a shopping centre!
No idea if any of this planning will work, but in theory it makes sense to me!