Relevant to this article are the other non-sexual parts of your character's anatomy:
As the character generation text states so clearly - gender is entirely a function of what you have between your legs.
Neuter: You have neither a vagina nor a cock
Male: You have a cock, but no vagina.
Female: You have a vagina, but no cock
Herm: You have both a vagina AND a cock!
Your body score is primarily influenced by the selection you made when first creating your character. There actually isn't all that much you can do to influence it later, besides using the Body Builder Perk. This number is also used in determining how much weight you can handle during pregnancy and when you have particularly large privates.
Note: The minimum possible value for Body Score is 5. This tells me we will eventually have more things that influence this value.
The initial values are determined by your choices when creating a character. Below are the options you have when creating a character and the associated Body Score.
Note: The amount associated with "Average" for males and females is different, thus the second entry.