You have to get past the mental part. Men thing they're too small. Women think they're too fat, boobs are too small, etc. Be you. Be confident. The self criticism is a downer. When I meet a woman and she mentions her "big ass" or "flat chest"...I immediately look for the exit. D-R-A-M-A.
Guys, here is a shock...there is someone who has a large dick than you. Yep. I know. SHOCKING! There are also guys who have bigger muscles, better abs, a better jaw line, make more money, whatever. Yeah, you're average. Get over it. You're in a situation where another man's wife is going to get naked and want to fuck. Open you're eyes...she's right there! Please her. Work on your oral skills. Work on your fingering skills. You lick her to orgasm, then gently finger her, tease the g-spot...make her cum again via the g-spot while her clit recovers.
And ladies....yeah, your boobs are probably small, or large, or funny shaped, or something. Your butt...probably bigger than that woman's over there but smaller than that other woman. No one cares. You going to drop your clothes, get that guy wet with your mouth and then ride him for an hour? You going to lick the other wife while your husband pounds you from behind? At the end of the night, is everyone going to be catching their breath, thinking 'that was freaking awesome'? No one cares about your boobs or your butt.
I'm not the most experienced guy. I've been around a little. The most memorable sexual experiences were not with the hottest women. It wasn't the thinnest or the one with the largest boobs. The most memorable are the ones where she had no hangups.
If you have a small dick, pick up other skills. The best pitcher isn't the one with the best fastball. The best pitcher has 3 pitches that he mixes well, keeping the hitters off balance. Bring your fingers, mouth and dick. Three pitches. Learn to mix them up, keep her off balance. When she heads back to the bench you want her smiling and thinking 'damn, that guy has some nasty stuff!'