The average erect penile length seems satisfactory. The measured size also depends a lot on the pubic pad of fat. If you’re overweight, it is advisable to lose some weight first.
You can undergo further evaluation at your local sexual health clinic/urologist.
Sexual intercourse is not just about penetration but a whole lot of other things. If you feel extremely distressed by it, visit a doctor.
Next Steps
Visit a urologist/sexual health clinic/psychiatrist
Health Tips
Further evaluation required to rule out hypogonadism