This video explains how to measure yourself for a contact staff. Generally, you want as long a staff as you can safely and comfortably spin without hitting the ground. A good way to start is to measure from the ground to the midpoint between your lower lip and the bottom of your chin. This is a good average contact staff length, specifically sized to your body.

Longer staves will spin slower and keep spinning more easily (due to angular momentum), but their size may interfere with certain moves. Shorter staves will both spin and stop spinning more quickly, but their smaller size opens up more possibilities.