From the Harry Potter book: "Mrs. Dursley was thin and blonde and had nearly twice the usual amount of neck, which came in very useful as she spent so much of her time craning over garden fences, spying on the neighbours."

In this case that means that her neck was very very long, and that she used it to be nosy and peer into her neighbor's yards.

This is a phrase that you would not normally hear as a description in common speech.