I have an XS and yes I'm petite — the coat fits quite generously — I would say 1 size up from usual size should give similar effect Hope this helps!! (figtny.com)
I ordered 1 size up from my GH usual size which worked. (garnethill.com)
The size charts suggested a size 16 for a 33 inch waist, so I sized up from usual size 14 and found it did actually fit. (missamymay.com)