There is definitely more than one answer to this question, but I would consider the following:
Chief's physical description of the nurse, particularly of her chest. She has very large breasts, which are obviously symbolic of femininity and motherhood. However, she goes out of her way to hide them, along with any other type of "softness."
Chief's hallucinations. Chief sees the nurse at one point as a huge monster who can reach out and grasp people with her arms. He believes that she has a bag full of mechanical parts that she uses to make alterations/installations in the patient's minds.
Her relationships with men. It seems that the nurse is in control of men in the text (even men like Dr. Spivey, who rank above her). What could this signify given that the story is set in the 1950's?
Her treatment of non-conformists. If a patient doesn't fly right initially, the nurse finds a way to make him conform. She ultimately destroys Macmurphy, the only one who is truly able to resist her.
If you consider all these ideas together, you'll probably figure out what the nurse represents.
. Nurse Ratched is symbolic of the combine and the system. She is not those directly but holds the power to them. The idea of her getting bigger and bigger is shown through the text. The patients view her as a ferocious wolf. Harding explains, “The rabbits accept their role in the ritual and recognize the wolf as the strong”... “Mr. McMurphy my friend I'm not a chicken, I'm a rabbit. The doctor is a rabbit. Cheswick there is a rabbit. Billy Bibbit is a rabbit. All of us in here are rabbits of varying ages and degrees, hippity-hopping through our Walt Disney world. Oh, don't misunderstand me, we're not in here because we are rabbits-we'd be rabbits wherever we were-we're all in here because we can't adjust to our rabbit hood. We need a good strong wolf like the nurse to teach us our place.” Coming from Harding, a non mentally-ill patient one with a relationship crisis is quite significant.