Tutu sizes?

For a tutu made for a child that wears size 5 how big would the waist need to be? I want to make one for my niece but want it to be a surprise for my sister cuz she doesn't know I'm making it, and they live in a whole different state so I can like measure her or anything. Lol. Anyone know what size I should do it? I really appreciate any help you can give!
Thanks bunches!

6-12 Months Waist: 16-18" Length: 8"
12-18 Months Waist: 17-20" Length: 8"
18mo-2t Waist: 18.5-21" Length: 10"
3t-5t Waist: 20-23" Length: 10"
5-7 Years Waist: 23-25" Length: 12" 8-10 Years Waist: 25-28"Length: 12"

Thank you so much!

heres an idea-http://www.skiptomylou.org/2011/11/01/no-sew-tutu-tutorial/