These lovely little skirts are cut in the School of American Ballet (SAB) shape, and becoming really popular outside of the US. 
 
Made from lycra/spandex, they are very practical, pull on, and the cut really lengthens the leg line. Perfect if you like your skirt short: very short at the sides, a little longer at the front and longest at the back. The *full* SAB skirt is an elipse shape and gorgeously springy when jumping! 
 
For a less full shape, have a look at the slimline alternative
 
Here are the approximate lengths according to waist sizes (excuse mixed cms and inches!!) Or let me know your preferred length. 
 
Waist 20-23" = Front 21.5cm | Back 24.5cm | Side 17cm
Waist 24-27" = Front 26.5cm | Back 30cm | Side 20cm
Wasit 28-30" - Front 32cm | Back 37cm | Side 26cm
Waist 31-34" = Front 34cm | Back 40cm | Side 28cm