I won't lie. The fact that the guy has a big cock may be big turn-on for your wife. But rest assured that this is a novelty. Got any toys in the toybox? If you do, I'll bet you didn't go out and buy a dildo or vibrator that was smaller than your own equipment. Part of the fun of it is the fantasy or just the sheer size of the thing. It's a challenge
After the fun, the toy goes back into the toybox and she doesn't feel any differently about you. Now this might not seem like a great argument because I'm either relegating your playmate to the part of "walking dildo" or trivializing the fact that it's NOT made of silicone, but it's a very real cock...attached to a human being. I guess all I'm trying to say here is that the 'big package' thing is merely a novelty. It's on the gotta-try-it-before-I-die list and some women find they love it, some can either take it or leave it, and other women would rather not do it again. As a woman, I can tell you that even if the guy had an absolutely perfect package and knew how to use it, I
know that the experience would pale in comparison to the fun that Mr. and I would have after we're back home in our own bed!! Why? Because no one knows me like he does. No one gets past my emotional barrier except him. I make love to him, and the more I enjoyed my experience with the other guy, the hotter it makes him. And his arousal is contagious. It turns me on like a 1000 watt light bulb to know that watching me with this other man has this kind of effect on him.
This is why it doesn't matter about size. The only time it actually matters is when you don't have an emotional connection with your partner like this. That's why the 'big cock' is often a prerequisite for male play partners; if they didn't have that...well their personality is only going to get them so far. There needs to be physical chemistry. That's why I'm really not jealous when another woman is more beautiful than me. She needs to be to hold my husband's attention because that's all she's bringing to the table. She can't offer him the same kind of closeness that I can.
Another thing: I actually prefer a medium sized, more manageable penis. The monster cock is kind of a visual novelty, but otherwise it's nearly unusable. Mr. bought me this HUGE dildo for my birthday one year. When I say huge, I mean it was shipped in a box big enough for a VCR. Now where the hell am I going to put that?? Ok, I've managed it a few times, but I had more than couple of drinks in me to help things along. This is a little bit TMI. Sorry
Bottom line: don't worry about it. Just relax and enjoy your wife's pleasure. Enjoy watching her discover another man's body. If this is the only other man she's been with besides you, think of how exciting this must be for her. Think of how lucky you are! She's sharing this experience with you. It's a very unique and intimate moment between the two of you.
Hope I've helped some.
Good luck tonight!