If your marriage is of the quality you believe this will work itself out in time.
The first requirement is time for the emotions to settle. If we ever find an effective way to deal with self-image issues like this, somebody's gonna make billions writing the book.
If you tell her she's not, um, larger, she thinks you're lying and bullshitting her.
If you tell her it's not an issue, she thinks you're nuts and bullshitting her.
If you tell her she's right about her observation, she thinks you no longer find her attractive.
How am I doing so far?
I think there aren't many options. You're right, you can't take back what you said, but I also think it wouldn't have mattered what you said in the first place. This isn't about your opinion of her size, it's about her opinion of her self and her stress over what the others will think of her.
Logic is probably not going to help.
You know your wife and your relationship very well... if you accept that you are not the problem, how have you guys handled things like this in the past? How do you get her to redirect her anger... or put it in perspective? Or in similar situations has it been better to let it blow over?
It might help some to show (perhaps subtly) her that you think she's the best both in the context of the trip and in general. Anytime there's discord, it's always best to return to the things you do agree on and feel the same about.
If she continues to brood, I'd ask her what she would like you do. Watch your tone of voice and be very genuine. It can be very disarming when you "force" the other person to help you solve the problem. If she says, "I don't know." Stay calm, keep your voice measured and say, "Well, that's the problem I have. I don't know either." Now you have something you agree on and you can build from there.
