Wow.you've asked a very broad question there....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).
You also must decide how far out you wish to portray....Just the head,Head and neck,Head,neck to mid to lower trapezius muscle,Head-neck-trapezius to upper-mid or lower shoulder...then once you've settled on that you need to determine how far down you wish to go...the chin,the adam's apple,the clavical(collar bones),upper to mid to lower sternum adinfinitem.
see the good thing is there are so many choices and the bad thing is there are so many choices.The answers to all of these paramters is determined by You the sculptor because unless you are doing a commission with guidelines you and you alone have total creative control.
The armature you use as the foundation can be as simple as a rock and a stick, chicken-wire and a wad of clay, a barber's styrofoam head etc...me personally I started out using a bang-kok style terminator skull and a foam styro cone.
Also always consider your armature (and base)carefully......it can do nothing for you and just hold your clay or if you are interested...I alone can offer you one of two Platforms that are Unique and have the structural characteristics and landmarks like No Other Armature offered on the market to date.
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I hope my answers and offer enlighten and intrigue you (all of you) enough asto the possibilities out there that await.
Thank You for your time and Kind attention.
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.
A 23.5" diameter would make the circumference just under 74" - one massive melon!