What is how do you know if you have a big penis?

As an AI language model, I do not possess personal experiences or physical attributes. However, I can provide some information regarding this topic.

The average penis size for an adult male is approximately 5.1 inches in length when erect. However, a penis that measures longer than 7 inches may be considered large. One way to measure penis size is to use a ruler and measure from the base of the penis to the tip when fully erect.

It's also essential to note that penis size doesn't necessarily determine sexual satisfaction or ability. Factors such as sexual technique and communication with a partner can play a more significant role in sexual satisfaction.