Does size matter in a fight?

This girl at my school is going to jump me on Monday. I told my mom and she told the school, but they said they can't do anything until something actually happens. Until then, it's just a rumor. I used to be friends with this girl and then she went and said my mom was fat and balding so I quit talking to her. Then she just got all mad at me for not talking to her and she's very serious about fighting me. She's about 5'5" and maybe 200 pounds. Fat, not muscle. I'm 4'11" and 105 pounds. I'm really worried because of the obvious size difference. She's also been in karate for about two months now, and is a white belt. I have no idea which style though. I'm in ITF taekwondo though, and have been for six years. But I've read a lot that taekwondo is terrible for self defense, so it seems it won't be much help. I go to a really traditional school too. It isn't a mcdojang. After six years I'm only a 4th gup blue belt and I go to class four times a week. Anyway, will her size just have her at a complete advantage? Do I have no chance even with my taekwondo training? Other than sparring, I've never been in a fight. I don't want to be in a fight, but if it's self defense there isn't anything I can do. Do I stand a chance at all?