Manufacturers size charts vary and there is no standard among them. This means that size 4, or size 6 is not the same from one manufacturer to the next - - just like purchasing a pair of jeans at the mall.   So if you're a size 6 with manufacturer X, you might be a size 8 with manufacturer Y.  If you buy from manufacturer Y thinking you're a size 6, then you will own a dress you can't fit.  
 
To avoid the sizing problem, always look at a manufacturer's size charts. The Bust, Waist, and Hip measurements for each size are itemized. To find out what size you really are according to that manufacturer, have your bust, waist, and hip measurements taken by the prom or bridal consultant. Then look them up on the company's sizing charts together and try on a gown by that manufacturer to see how the sample fits to your personal figure.  Manufacturers make their gowns using European sizing and not we are used to when purchasing clothing from Sears, JC Penny, Old Navy, etc.
 

A final note: when special ordering your gown, it is always best to buy the size that corresponds to the largest of your bust (and cup size as most are made for b-c cups), waist, or hip measurements.  In other words, if you're a size 36 bust and a size 26 waist, and the size chart says size 36 bust equals size 8, but size 26 waist equals size 6, then get size 8. You can always have the waist taken in afterward.  The problem is that you can't always have the bust, or other dimensions, taken out because there might not be enough fabric or seam allowance provided.  If you are under 5 feet 9 inches tall with shoes, you may need a hem to shorten your gown.