The length you go with is entirely up to how long you want your hair to look. 

There are a couple of handy tips to know though: 

⭐When in doubt, go with the longer option. You can always cut the hair down to the length you want. But you can't add any back. 

⭐It will be easier to blend extensions that are close to the length of your hair, or just a little bit longer. That doesn't mean you can't blend longer extensions if you have short hair. 

⭐The two images below will give you a good idea of how long each  option is: