CARNAL NATION

Well shit. You already layed it all out: "all sizes are perfect in their own way;" size doesn't matter; it's what you do with it, etc., etc. You know what to say. The issue, then, might be standing your ground.

So you can take two directions. One: answer them with what you really think, and then refuse to entertain it any further. "Nope, I'm done, I already answered. It's not a brush-off, it's the truth. That IS what I really think." Don't let 'em push you on this. Just answer, and make it so matter-of-fact that there's nothing else for them to follow up and keep pressing you about. Or, secondarily, you can play therapist for a minute and delve into the deeper patterns of insecurity. Why does it matter to you? What difference does it make? Maybe there's a way to discuss some of the cultural implications of this, too—like the value that culture places on "big dicks" and why; what makes that problematic; where this comes from; how to have more security about your sexual abilities regardless of dick size.

This is not 'after-school special' of you, but it might be very time consuming. And it might not really be very effective. If I were you, I'd probably gauge whether or not the conversation would be productive depending on the guy. Some won't respond to any sort of deeper analysis of where this insecurity comes from; some might. Use your judgment, and keep your cool. If it's not an issue for you, don't let them bully you into placating their insecurities.