Someone seated at the same table at a wedding is going to be wearing a dress ...

I say wear it. Doesn't matter if it's the same color or not. I'm shopping for a dress to wear to a wedding this summer too, and it's incredibly hard for me to find something that isn't black or white. If you love your dress, stick with it!
 
Definitely wear it! Plus who's to say she won't change her mind and wear something else once she hears you'll be in an orange dress?
Rock your cage heels, killer bag, and fab jewels and no matter what you'll shine. Have fun!
 
I think the most important thing here is that your comfortable. Not comfortable in the dress but with yourself in the dress. There is nothing worse than being self conscious and it will ruin how you look. I always tell anyone that asks me what to wear to something, that these days you can wear just about anything you like as long as you feel good and are confident in it. So if you can put on that dress and feel great about how you look and feel, wear it. If not,find something else. As for the size issue, have you ever seen in the front of Us magazine the who wore it best section? The size 2 girl doesn't always wear it best, the confident stylish girl does! Have fun!