Legs covered to at least the knees. Can wear pants. Shoulders covered - they offer shawls at the entrance though. Elbows don't have to be covered. That's all. Just went a few weeks ago. Was amazing!
Women should dress modestly at the Western Wall. Dressing like an Orthodox Jewish woman is your best bet: arms covered past the elbows, skirt past the knees, shoes not sandals, and head covered too.
The minimum is to cover the legs and shoulders. You can wear pants and you can wear short sleeves and walk right up to the Kotel. They provide shawls at the entrance for those with bare shoulders. I think you can even wear shorts.
Women were not allowed to pray at the Western Wall of a very long time, only in the last 50 years have they been allowed. As such, there are no specific rules other than to dress respectfully.
