I am by no means an informed source on this but I'll open the dialogue, so to speak, with the (mis?)conception that a staff should be as long as you are tall.
I really should put a question mark on the end of that tho' - which should be a warning as to how 'sound' my comment is .
SukerKin is, from what I have heard, correct. My first instructor liked his staff to be about a palm's width taller than he was, though some tournament competitors like theirs a few inches shorter than they are (better for spins, etc.) so as tall are you are is a good rule to go by.
It depends on the staff system you're going to learn. Some use a "long staff" that only reaches to shoulder height, while others use a "long staff" that's goes up to your fingertips when you extend your arm over your head, and I even know of one system that uses a 12 foot plus staff!
Consult with your instructor to find out the right length for what you'll be taught. If in doubt, 6 ft or so is a good starting point, typically. Error on long, because you can always trim it -- but it's hard to add length!
Depends on the art you are studying. I have both bo and jo as I study both kobudo and aikido. 6' staves seem to be standard for open tournament competitions.