I am 5'9" and usually a US 8, sometimes 10, and the XL I ordered fits me great, leaving a bit of room for me in case I gain a few pounds, so I'd say the XL is a solid US 10 (UK 14), which matches the size guide in the Ebay listing.
I won't lie, so I'll tell you you won't fool anyone who knows a bit about fashion into thinking this is a real DvF dress. The craftsmanship and fabric are both alright, but definitely not DvF quality. But is it a good replikate worth your US$ 59? Absolutely!
You can buy the replikate here.