What size panties would a 13 year old girl wear?

Me and my friends wear boxer briefs and compression shorts all though we really don't need them when we hang out together we just take them off and compare each other's you know whats... well if your 15 you should try boxer briefs and compression shorts they are very comfortable and the hold your junk really well. The best part is when you go to bed and take off your pants under the covers...