We will only speak about the erect penis length here because one study showed there is no strong link between the flaccid length and the erect length.
In 1996, a study published by the US journal of urology (led by Dr Wessels) considered that a small penis measured less than 8 cm or 3.14 inches in erect state. This length has been measured with the bone pressed methodology. It means those 8cm doesn’t include the fat pad length as the measurement is taken from the pubic bone until the tip of the glans penis. Subsequently a man who has a 8 cm penis will not insert 8cm in a vagina because there will be also the fat pad length between the pubic bone and the skin layer.
So, i have my own technique to check if a man’s penis is small or not. Normally a palm’s hand measure around 8 cm from side to side. So, if a man is taking his erect penis shaft in hand and if he doesn’t see the tip of the glans it means he has a small penis.