I bought it , brought it home and threw it on right away. I hadn't tried it on in the store, since it was only $5, I figured if it didn't fit at all, I could just take it back and re-donate, or give it to someone that it might work on. I put it on as soon as I got home - and it looked great! One small problem though - I couldn't get it zipped up all the way. I was so sad, and I also couldn't understand why it fit so well everywhere else, and the last two inches just wouldn't zip.
So - I decided to give my local seamstress a try. She is so fantastic, and pointed out to me that the reason the dress probably ended up at the thrift store was because someone else had already altered the dress, and the alteration appeared to have gone wrong at the top. She said there was plenty of material at the top of the dress, and she could get it back in shape in no time! I was so happy (and also relieved that I don't actually have a really fat back). So, a few days later, plus a good dry-clean, and voila! A beautiful dress for the ballet!