Arula open back puff sleeve dress

I bought this dress and didn't know what B size was (I was thinking A was small, B was Medium Etc... )I thought it was a medium... it is not. It's a 2x. It's got.lots of stretch with the panel in the back and a tie. Also, it has pockets, which is always awesome. Like new, I bought it off another resale site, so it's not new, but it doesn't look worn at all.