Standard anatomical term; neutral in medical contexts but can be direct in casual speech.
from Latin
ペニスのサイズは個人差が大きい。
Penis size varies greatly from person to person.
ペニス is a neutral, clinical loanword for the penis, while ちんこ is vulgar slang unsuitable for formal or medical contexts.
Used for long cylindrical objects; this is the most common counter for penises in ordinary speech.
strap-on dildo
Loanword from Latin 'penis', borrowed into Japanese via Western medical terminology.