Ask her what her size is. When she asks why, just tell her you want to get something for her. You dont have to tell her what it is, but if she's anything like me, she'll hate if you are trying to snoop through her clothes, trying to find her size. Alternatively, ask one of her friends. :)
I don't know how old you are but do you ever have an opportunity to look in her dresser and find out her size? In my opinion it's not creepy to look in her underwear drawer since the purpose is to buy a gift. You could also get her a gift card and let her pick something out herself.