The specific length extensions can look different on different persons, the taller you are the shorter the hair looks on you, and vice versa. You can take a look at below length guide.
Here are some tips that might be helpful for your length choice:
You don't want to get the hair extensions of length that doubles your hair.
If you are new to hair extensions, you might want to try a shorter length to start.
You may do some trimming to the hair extensions or your hair for a better blending.
Get the higher weight set if you decide to get length that is much longer than your hair.