Hi Emerald;
You need to get a corset one size minimum below your standard dress size. So mine is Size 18. I was a a 22 Dress size, but I have dropped a dress size to 20 following nearly 4.5 months HRT, so my deportment therapist recommended a size 18 corset (with tightening this can bring me down to a size 16 waist).
I am not there yet though.
Hugs