The pattern suggests 5 to 6 feet. It would be better to err on the side of longer - if it hangs too low, you can always cut off the excess after knotting it on. I made mine about as tall as me (5'8") and it hangs kind of low on my shoulder still.
Hope that helps, and be sure to share pics when you're done!
Homeschooling mama to a 5 and a 7 year old!
Knitting for baby - those little things fly off the needles!