Smooth Layers Scoop Neck long dress sizing

Hi!

Hoping someone who owns the smooth layers long dress can give me a hand! I absolutely love the look of it and am happy to splurge on such a stable wardrobe item. Just a bit stressed about ordering the right size, I live in Australia so expecting shipping to take a while and really hoping to avoid hassle of returning/exchanging for a new size.

I am normally an AU size 6/25 waist/32A bra. I have the slip dress in XS and S. Both of them fit but I find the XS little bit too clingy on my lower body (narrow hips but biggish bum). Does anyone own the smooth layers dress and have some sizing advice? Struggling to find reviews of it online.