One Size Fits Most Which, in my experience, usually means One Size Fits Preschoolers lol But in this case, it apparently really means One Size Fits Most average sized ladies! I'm a size 6, size 4 in generous Kohl's and Old Navy stuff lol Size 8 in mean old Gap etc.
Not the most fiscally responsible strategy (but way more fun!) is to buy two: one that's in or just a millimeter beyond your comfort zone, and one that you wish you had the nerve to wear. Bring both and decide that night based on where you fall on the liquid-courage-ometer. (And the other you get to use as an excuse for another trip so you can wear it!)
This may be old news to most and hopefully it hasn't gone away, but if you use the URL: Sexy Lingerie Shop, Intimate Apparel Lingerie Store, Sexy Halloween Costumes you'll get 10% off your entire order. Dangit, changed what I posted! Enter www(dot)yandy(dot)com/wow