| | Re: tutu dress
Depends on the age/height. If it is someone that you know, take the measurements of where you want it to start (around her armpits and measure down) and then double it and then add about an inch for wrapping.
Example, if you want it to measure 12 inched long then double to 24 inches plus one inch, so you will cut 25 inches.
Hope that helps! (This is just the way I do it though)