Lucky for you pleasuring a female doesn't require going in deep into to the far reaches of the cave. Most of the pleasure areas like the clitoris is easly to stimulated during intercourse even with a small penis. Not that I am speaking from experience just assuming from my knowledge of the female anatomy.
Now, if you are REALLY worried about it I suppose you could look into things like male potency supplements, penis pumps ect. I wouldn't go down that route though except as a last resort.