I was taught in a very religious school that it could be three finger widths. Hair does pop out because we’re very busy, Baruch Hashem.
By the way, the snood is the way to go. There’s this Rabbi who just passed away who said in his will that he’s giving a special bracha to the women who wear snoods. I now feel really good about wearing snoods and I’m planning to buy some more! Chabad women should keep wearing their sheitels because that’s what their Rabbi told them to do.