There is no recorded average of a walk-in closet size; however, the minimum size for a walk-in closet is roughly 72 to 75 inches. This will allow clothing to have 24 inches on each side as well as 24 inches of walkway room down the center of the closet.
The height of a closet rod is also an important consideration when building a walk-in closet. The rod should be 44 inches above the floor. This calculation is based around the size of a man's standard shirt. When adding two rods to a closet, the top rod should be approximately 84 inches from the floor with the lower rod at 42 inches for best usage.