Sizing on Yonnex seems to run fairly true. I wear small tops and medium bottoms in most brands.
I think the thing with sizing for me is that the skirt part of the dress has to have some extra yardage, if you will. If it flares a little (as the pink Yonnex does but the red Bolle does not) I will do OK.
There is one issue with the pink Yonnex. There is maybe one inch of extra material around the top of the bust line in back, where the tag is. That leaves a gap that looks like it needs to be safety pinned. I'm going to take it to the dry cleaner tomorrow and see if there is an easy way to take it in a tad.
You can see from this picture that anything designed to look right on an upper body ripped like that is going to gap a bit on li'l ole me.