I saw it, too. I'm sure it's no accident they gave her heels that tall given the subject matter of the play. She was wearing heels that were easily three or four inches tall, which would make her as tall or taller than her co-star, Hugh Dancy. Here is the excerpt from the Times article (which I will link below).
"And at 5 feet 8 inches, with broad, square shoulders and large hands and feet on a thin frame, Ms. Arianda can instantly dominate a stage."
Article URL: http://www.nytimes.com/2011/05/28/theater/nina-arianda-a-rising-star-in-born-yesterday.html?scp=1&sq=nina arianda&st=cse

I just saw her on Broadway in "Venus in Fur". When she wears her character's spike heels boots, she looked taller than her 5 foor 11 inch co-star, Hugh Dancy.

She's 5"8 according to an article in the New York Times.