What matters in sex is pleasure more than size. A non-erect penis measuring 3-4 inches is considered normal. And an erect penis can measure 4-5 inches. But this is average It can vary. An even shorter penis can get erect and enter vagina or anus. It is more about what it does than its size.
Sex also involves acts that does not require the penis. Pleasuring each other is not limited to penetration. So think about sex in a broader sense.