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 the erect state. This length has been measured with the bone pressed methodology.
Take a ruler and place it above your penis. Press the ruler against the fat pad against the pubic bone and measure to the tip of the glans penis.
Subsequently, a man who has an 8 cm penis will not insert 8 cm 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.