You may be suffering from micropenis
The average penile length is 5.5-6.5 inches. The proper way to measure is to stretch the penis and measure from the base to the tip of the penis. Often times if the patient has a prominent or a bulky fat pad then this may lessen the penile length.
You may be suffering from micropenis and I would strongly suggest seeing a Urologist
for a proper examination and for him to advise you on treatment options. I would caution you on medicines and gels sold all over the Internet that claims that they can enlarge your penis. Please don't waste your money on them.
You may also try jelqing manuever. It is stretching and milking the penis from the base to the tip everyday. Just be gentle to avoid damage to blood vessels and your organ.
I hope I am able to answer your question clearly and satisfactory and Thank you very much.