Why does it matter? Let's put Jane in the female corner and Dick in the male corner.

Jane: Of course size matters, big ones are important for my satisfaction.
Dick: That's unfair. As a straight man I have no idea of whether my penis is relatively big or relatively small. There's no real way for me to find out, either.
Jane: That's hardly my problem, is it? If you're not big enough, you're not big enough. Nothing either of us can do about it.
Dick: If it were too big, would it hurt you?
Jane: Oh yes.
Dick: So, you want to be hurt?
Jane: No, I want it big, but not too big.
Dick: You want perfection?
Jane: No, I, well, I want something in the right area, at least.
Dick: Did you know that studies at the Kinsey Institute have proved that your vagina grows slacker with every lover you take? It's as if I have to compensate for your sexual history.
Jane: Just as your penis will grow larger with age, but lessen in potency. What are you getting at here?
Dick: I don't know. I just feel that I'd be acting like a sexist prick if I told you that your breasts were too small or that your thighs were too fat, but you seem perfectly entitled to tell me that my penis is too small. How does that work?
Jane: That, Dick old son, is just the way it is.

I apologise to all devoted Dick and Jane readers for the absence of Spot the Dog, but he didn't really have much to add.