The recommended length is very much dependent on the style you are looking to achieve. For longer styles its best to get the longest length available and similarly for the shorter styles a shorter length.
Good to know: The listed lengths on the product page are folded lengths eg. 24" unfolded is 48" in total length.
Hair Length Guide
The length of the hair you select will always be based on the straight length of the hair. However, just like with your own coils, the curled up nature of the strands masks the straightened length of the hair. Depending on the tightness of the curl, the hair can between 10-70% shorter than its straightened length. The tighter the natural curl pattern of the hair, the shorter it will look at any given length when compared to straight hair.
*approximate shrinkage according to curl type.