A wand's length is in proportion to the owner's height.
Examples:
Harry is average height, and his wand is 11 inches.
Hagrid is enormous, and his wand is 16 inches.
Voldemort is described as tall, and his wand is 13 iinches.
Umbridge is very short and her wand is described as unusually short.
More evidence to back it up: Mr. Ollivander does all those measurements before selecting wands for you to try.