Maggie. I recently had exactly the same query. I had considered myself to be a 12 to 14, maybe even a size less for US rather than UK/Aus sizing but measured myself and found I had a Lisette/Simplicity size 20 hip measurement. I erred on the side of vanity, ignored the tape measure, made the size 14 and found it to be far too small around the hips (only just salvageable by sewing a very, very narrow seam). I treated that dress as a muslin and have decided I’m a size 16 up top and 18 around the hips….
I don’t know when a muslin stops being a muslin as I’d need to test the shoulders, bust, waist, arms and hips and could possibly be up to three different sizes across all of those points. I think a trial run of the whole dress in cheap fabric is a great idea! Good luck