First, deeming any one specific face shape to be 'ideal' is small-minded and absolute. Second, attempting to restrict one's choices due to a preconceived notion of beauty is wholly inaccurate and completely unnecessary when paired with a mature and refined skill set. Hairdressing achieves a win if one feels beautiful. Period. There is no best style or length for any one individual. There are likely multiple options that can all be perceived as both attractive and flattering as long as the work has been executed well. That said, some of this perception will always be considered subjective. One's confidence level and sense of self often plays a part and effective communication, to begin with, is absolutely essential.
Most clients have a really strong idea of how long or short they want their hair to be and the general aesthetic sense they wish to convey to the world. The most important factors to consider when choosing a hairstyle is the texture and density of the hair, the desired maintenance regime, and how frequent the person wishes to maintain it. That's it. With adherence to these principles paired with thorough consultancy, the odds of success and smiling faces is often great.