Yes, as someone noted, first-- have you talked to all the other bridesmaids and has everyone checked that the right dress went to the right person in the wedding party?
If this is "your" dress: This sounds so very specific that you will end up getting an ulcer as you try to match the designer, the color, the fabric, the size all on your own.
The business that sold you the dress needs to fix this -- not you! Why haven't you called them immediately, told them the problem and then TOLD them (not asked, told) to fix it immediately? Did you contact them and they said there was fabric to let it out and that was the end of it? Not good enough on their part. Get back with them and be very assertive.
Ask for a supervisor. And if he or she is snarky or balky, be clear that as soon as you hang up the phone you will file a complaint with the Better Business Bureau AND the consumer affairs bureau in the state where the business operates, and mention that you will be telling other women not to use them because they don't provide acceptable customer service. Then ask if they'd prefer to fix the issue....?
My friend had a problem where a bridal shop insisted she had to take a gown she had ordered that had a stain on the front when it got to her. She walked in with stamped letters in her hands, addressed to the BBB and the consumer office and said, "You can fix this issue, or I walk out of here and mail these." They got very helpful--very quickly. That was a while ago; these days I might add, "I can mail these, and then I can go on Yelp and other review sites to mention this business's level of service...."