Traditionally, a staff should have the lenght from the ground to your arm. But You can use the length You feel comfortable with.
Its better if you make one and not get a ready-made one.
When you walk down the woods or when u cross a tree, You will know that this branch if for you to use it for ur staff. It will come to You.
All You have to do is to carefully cut it and carve it the way you want it to look.