How can I overcome the small size of my partner's penis?

For you or for him? He's probably quite self conscious about it & his ability to satisfy you & thus needs assurance that other aspects of intimacy are more important than inserting a large penis. At least 50% of men wish they had a larger & majority are within normal limits for size. He should be able to bring you to a sexual climax by stimulation of clitoris if necessary. Mental & spiritual relationshid more important.