The first one stayed for a week, but that was primarily because they thought she had... developmental problems. It turns out, it's just because her father is funny looking, so it translated into the baby looking like she had several markers for Downs. The specialist looked so embarassed when they realized what the reason for the markers was... The second baby was set to come home on day three, and then day two she had a collapsed lung (which we believe was due to something the nurse did actually, but can't prove...) She ended up coming home on day 5. And then baby three actually never went to the NICU at all other than a few hours of observation at first, and then she roomed in. She went home after three days. All three of them were fairly big for 35 weeks (in around 5.5lbs), so i think that was a contributing. My SIL has a bicornate uterus, so they weren't surprised she delivered early, and I'm sure that factors into it all.
The newborn ones DO seem small, but they seem like they'll easily fit up to about the 8-9lb, which is when baby should fit the smalls anyways. All the newborn ones I have were either free, or I got the kushies ones I think all four for $2.99 or something like that at the thrift store. So I'm ok if they don't get a lot of use... But yes... planning on probably buying more OS or meds once the baby is here and growing into the first OS I bought so I'll have an idea of fit.