Don't worry about this at all, you are fine! The average size of the erect/aroused penis ranges from 3.2 inches to 6.5 inches, but even lower sizes don't make a difference to the ability to have sex, please one's partner or have children.
The aroused female vagina only extends between 3 to 4 inches, and even here, the most sensitive parts are towards the outside. Girth is a little more important, but most important is how you use it.
The only way to increase penis size or girth(thickness) is through surgery, but this is painful and risky. None of the medicines marketed to increase penis size work and they are a waste of money. We don't recommend surgery or any medicines.
Focus on the person you are with and make sure your partner enjoys the experience - there is really nothing more important than that in sex. Anyone who tells you that size is important doesn't really know what he is talking about.