Mint Open Back Halter Dress

Mint open back halter dress from Tobi. Never been worn, I ordered it online (size medium) and it was WAY too small. Tobi's website says that a medium is for sizes 6-8, for me it might as well have been an extra small. I'm 5'8, 130lbs and my butt is slightly on the bigger side. Sizing as a Small. Price is 100% negotiable (as long as it's reasonable) and I'm ready to send it out ASAP. NWOT