If you have a narrow face, you'd better choose the small size of sunglasses. And the square framed sunglasses can fit your narrow face well. You can go to any optical store to choose one of these sunglasses and try them on. Then you will find they look very good on you.
I think square shaped sunglasses will look good on a narrow face. But the best way is to try different styles of sunglasses to find the most suitable one. Remember to bring a friend with you. She can give you some opinion about it.