"So, normally I wear a size 12."
"Ok, so depending on the gown you could where between 12 to 16."
"WHAT!?! I'm a 12 - sometimes a 10 - NOT a 16!"
On average, we have this conversation twice a day with women of all shapes and sizes. Brides, bridesmaid, grad, mother of the bride - if you're wearing formal wear, expect to go up at least one size, maybe two. I've read and heard varying reasons for this- bridal sizing hasn't evolved with women's changing shapes, sizing hasn't changed in 80 years ago, etc, etc. Whatever the reason, just be aware that formal wear and ready to wear are going to fit you differently. If you can do this, instead of your self-esteem taking a dive, it will soar when you see how amazing you look in a well-made garment.
Keep in mind:
Corset (lace up) backs are always more forgiving then zipper backs
Size will change from designer to designer
Each garment is different - is it fitted through the hip, or more A-line?
Although I've never seen it (and I can't bring myself to do it - yet) I've heard tell of some studios and salons that actually cut out the sizes, so that ladies have no idea what size they are wearing. The focus can then shift to how a woman feels in the gown, rather than how the tag makes her feel.
Maybe there's something to that.