Fillyboo Angel Maxi Dress - Flowerbomb Lilac

I ordered an XS and should have gotten the small but I missed the exchange window. The dress still has tags on it and has not been worn. I am a size 4 leaning size 5, but size 6 is too big. So I think the dress is a true XS but does have give because the waist is elastic. https://www.fillyboo.com/products/angel-maxi-dress-flowerbomb-lilac