I'm HUGE for a ballroom dancer and yet have had a reasonably succesful amateur competitive career dancing against people younger and much thinner than myself (for reference, I wear a size 14, but used to be as large as an 18).

Most of the used dresses I see for sale fall into the 4-6-8 range, so if you can wear any of these sizes you are doing fine and should just focus on your fitness level rathern than on how much you weigh. And heck, if you're larger, like me, don't stress yourself out about it. Just focus on being strong and healthy and being able to dance all five dances full-out without collapsing at the end :)