I think you may have confused diameter with circumference. Even then, a 23.5" circumference would not be considered quite small. I think I recall reading that 22.5-23" is considered average for an adult male..If you are talking in terms of "human" the diameter of the cranium(the head) runs 'on average' between 23.5"(quite small) to 25"(good average).
But that would require time and work. You can't expect either of those things from someone who starts a thread with a 1-line question with no question mark.I'm always puzzled by questions like this placed on the internet. You have a head on your shoulders, you have friends and family. Get the tape measure out