The answer might be pretty mundane. The entry seems to simply be an estimate made by the registrar at the time of registration:
Question 1.—Tall, medium, or short (specify which)? Slender, medium, or stout (which)?
The answer in each case should be a single word, as "Tall," "Medium," or "Short." Do not try to give additional information.
Question 2.—Color of eyes? Color of hair? Bald?
State whether eyes are gray, light blue, dark blue, light brown, dark brown, or black. Color of hair should be given as light, light brown, dark brown, black, or red. Whether bald or not should be answered simply as "Yes," "No," or "Slightly." Do not add anything else.
See Registration Regulations (Washington: Government Printing Office, 1917), p. 19. (Included in Acts and Resolutions Relating to the War: Passed During the Sixty-Fifth Congress, First Session... (Washington: Government Printing Office, 1917), p. 111.)