I had twins at 34weeks weighing 4lbs 3oz and 4lbs 6oz.. they went home before I did. (48hours) it depends on how developed your lil one is.
That depends on just how small she is as well as how well developed her systems are. If she's less than five pounds then yea absolutely she'll have to stay until she gains some weight but if she's over that then they just have to make sure that her temperature regulation, blood sugars, heart rate, respiratory system, and some other systems are all within normal limits. Some IUGR babies have a really hard time adjusting to the outside world and some do great as soon as they're born. They're obviously keeping a close eye on you and are watching our for your baby so you know she'll be in good hands. Try not to stress too much - there's nothing you can do right now but stay as healthy and stress free as you can for your baby's sake.