I agree with
@Catbird9. Tell her the size you have and if she wants it, send that for the winning bid price.
Since she rejected your request for cancellation, there's nothing you can do. You can either confirm that she wants the size 12 and you'll send it or you can just wait to see what she does. (She won't be able to file a dispute for non-receipt since she didn't pay.)
She can't force you to sell an item if you don't want to sell but she can neg you.
I looked at the listing and have a suggestion for you should you choose to relist it with the correct sizing information.
1. Since it's described as "NWT," include pictures of all sides of the tags. That would also show the size, confirming both to you and to buyers that it's listed correctly.
2. Show a picture of the size tag attached to the bra. This is another confirmation that it's listed correctly.