Picking length of a staff?

Some say staff length should be just under your shoulder height.
If doing Martial Arts staff fighting moves it should come to your forehead height.
It is really about personal preference on how long a staff is.
A shorter Staff
They are short and light usually, much easier to transport, easier to carry around to practice. Very good for baton style moves. Lots of little spins around the place. Good for doubles when you are learning, not so good for contact, because they are not very heavy, but it is possible. They do also need to be pushed a lot which means to keep the spin going you need to exert a lot more force and they are easier to burn yourself with as there are flames really close to you the whole time. It is also hard to hurt yourself when practicing if you whack yourself there is not a lot of kinetic energy behind it. It is not going to bump and leave a bruise as much as a long staff. A short staff is generally shorter than twice your arm length.
A longer Staff
They are harder to transport unless they break down. They are really good for speed and for contact moves which roll around the body. The weight allows it to have its own momentum. It is harder to burn yourself, but much easier to whack yourself and leave bruises. The length of the staff can limit the number of moves you want to do, particularly if it is going to keep hitting the ground. So it is particularly a good idea to not have it much taller than yourself, but there are lots of people who have it taller than themselves and really enjoy it. Ground to shoulder height is a good length to start with.