Most of the nerve endings are in the first 2 inches going in. 2.5 inches is the minimum to get the job done. I think the average was around 5 inches.

Just as the penis can vary in size, so can the vagina. The ideal length of the penis varies based on the size of the vagina. Sometimes a "below average" penis length will be "just right" if the vagina is unusually short and even an "average" penis will be uncomfortably large.