Supergirl is nowhere close to 6'0 though, thats too tall.
Anyway, I think Mischa's on screen sister on the OC, Willa Holland, resembles the teenage Turner Supergirl more (and is currently the right age for it).
Problem is, like Barton, she can't act beyond playing a valley girl. Personally I dont care about that but I know the fanbase will rip it apart.
Kaley C would be awful though, you need someone like her: