After I found out the book would be made into a movie, Anna Sophia Robb became my pick for Pet/Wanda. She is a perfect fit! She is the right age (16 right now and would be 17-18 during the filming), petite (5'1-2"), blonde, fair, silver-ish eyes, and very angelic looking.
Other choices that I have come across just don't fit the description so well.
Taylor Swift is 5'11, entirely too tall for Pet/Wanda. Plus we don't even know if she can act.
Emma Ward is also too tall (5'10).
Elisabeth Harnois fit the look when she was a little younger but now she is too old (30 years).
Amanda Michalka is too tall (5'7).
Emma Roberts is the right height and age, but her hair looks so fake when its blond.
Dakota Fanning would be a contender if she wasn't in the Twilight Series and this movie will probably film at the same time as Breaking Dawn.