Let's say you want the tutu to be 14" long. Double that length to 28" and cut each strip to 28" long. You determine length.:)
If using tulle from the bolt, the best way to cut strips is to leave the tulle folded as it came from the store and cut it down to 6"strips so you're cutting though multiple layers. Saves you time. Or purchase 6" by 25 yard spools to bypass cutting the tulle into strips. You will need between 1-3 rolls. Average toddler tutu will use between 1.5 to 2.5 rolls.
Some strips might not come out completely even, but it really is okay once all strips are tied on. Any mistakes can be hidden.