This is totally up to your own discretion. They definitely don't want pages so it's up to you on how to structure the written exercise. It's difficult to put an exact word count/page length because certain case studies may be more complex than others (and so may require more content or have more points that may affect the client) than others.
Either way, keep asking yourself: (1) is it relevant to the task given? (i.e. if the task is for a real estate client, is the content you're writing applicable to them/do they need to be aware of this or is it more applicable to a banking client) (2) are you being succinct/to the point? (3) is the format of your written document easy to read and easy to digest.
Good luck!