The dress size you would wear if you are a medium-large would be 10-14. It could be larger or smaller than that, but that is an average. A ladies size 10 is supposed to fit hip sizes 34.3 to 35.8 inches, and a bust from 32.3 to 33.9 inches. Size 12 fits hip sizes 35.8 to 37.4 inches, and a bust from 33.9 to 35.4 inches. Each store seems to have their own size system. A size 10 may fit you perfectly in one store, but is way too small in another store. It is not you but the size system used by each store.
id say if the dress is tight go for the size that fits ur larger part and u fix the rest. If the dress is loose at hips or top you can go with the size that fits the area that the dress is tight on. For instanse i have a medium top size and a large hip size. if dress is loose on bottom, i can go to a medium as hip dont really matter. hope it helped.