What is my dress size in numbers?

I wear a small in almost EVERYTHING... because i like my clothes really tight on me or form fitting... and I just recently joined a modeling agency and they asked me for my dress size in numbers but IDK what it is... My prom dress was a size 6 because I am VERY busty (34DD to be exact)... the retailer tried to put me in a 1 or 3 but I had to go up extra sizes so that my boobs would fit in the dress... and finally we found out that I could look form fitting in a size 6 but only in the mermaid dresses... Am I a six in dresses or one... I'm confused... Also they asked for the dress size in inches... How do I figure that out?