"If the girl is fit, you must acquit!"

the argument that won her the case was her disrobing and declaring that she could not possibly be guilty because the gods gave her this body.

You might be thinking of the painting Phryne Before the Areopagus. There's a lot of variations on this painting. The general story is a man removes the clothing of a woman (usually being accused of sorcery/impiety) in front of a judge/jury in order to sway their opinion. Or to "prove" that her beauty was from God/gods and not something she bestowed upon herself with sorcery.

It's a story about Phryne, she was courtesan in Greece born around 370 BC, an actual person. We don't know much about her life other than she was self made and one of the richest women in Greece at the time. She also happened to be very famous.

This version of the story is likely a dramatization after the fact. Where she is brought on trial for impiety by a man known for frivolous trials. She bares herself and beauty is seen as a blessing of the gods and is acquitted. May or may not have actually happened but it's a good story.

We do know that there was a trial, there is an account by the man who spoke in her defense, Hypereides. We also know she was beautiful and popular enough that she was able to charge very high prices even in old age.

She is also said to be the model for the original Aphrodite of Knidos statue. Which was destroyed but there were of boat load of copies made in antiquity.