Final answer:
The diameter of a hair fiber is the same as the inside diameter of the hair follicle, not the size of other related structures such as the root sheath, hair matrix, or arrector pili muscle.
Step-by-step explanation:
The diameter of a hair fiber corresponds to the inside diameter of the hair follicle. Considering a human hair has a diameter of about 60 µm (6.0 × 10-5 m), it fits within the hair follicle which is the structure that the hair grows within. The hair root, hair shaft, and hair matrix are related structures, but they do not directly match the diameter of the hair. Specifically, the hair matrix contains a layer of basal cells which give rise to the hair and the inner root sheath, but this matrix doesn't determine the hair's diameter.