Necklace lengths usually are as follows:
15-16" is a choker length
17 -20" is a nice length for most women
20 -24" is nice length for most men
Of course you may order any length you prefer. I like to wear a few chains at once and often have a shorter 17" one, then a 21" and a 25" chain on...that creates a nice look each chain sitting inside the next.
I also encourage you to add a bracelet chain to the back of your necklace if you would like to create a longer necklace depending on your preference.