There are 2 main rules you need to keep in mind when buy extensions:
1-find a salon that is known for doing hair extensions
2-only buy 100% human Remy hair
The only type of people I would say shouldn't use this type of hair extension are those who have very fine and sparse hair, extensions can cause breakage and inhibit hair growth.
Now, when it comes to length I buy the longest possible which is about 18-20 inches. You can buy longer than this but it wont be Remy quality and it wont last long! You can see in the picture i posted where my hair is layered and barely touches my shoulders that this length works with short hair! You don't have to cut it or layer it to match, it blends and looks natural.
Hair texture is important! After reading this if you still have any questions the people at the salon will help you, I always ask for help (that's why i know all of this) and they want business!
If your hair is fine and isn't too thick naturally a Silky texture is what you want:
The Yaky texture is what I use, my hair is naturally thick and curly so when it's straight the shaft is thick:
And when it comes to color you should really consult the people at the hair salon on which color you should get, they have the experience and know the brands. Luckily black is easy to match! But this is also another good reason why you should buy from a salon:
a. they know the products well
b. if the color isn't what you want they will get you something else!
When it comes to what brands you want, all the above pictures are from the Bobbi Boss brand which has a Remy version and non-remy. All my extensions where from this brand.
Price? Well buying from a salon can be more pricey than the internet mine have costed about 130 per packet (ream) but i support buying from local salons which is important and i love the hands-on help and how they make sure i get what i really want.
The salon i buy my hair from is Blessings salon in Eugene Or. It's a salon that specializes in ethnic hair types (i'm mixed so my hair is very curly) and extensions.