I bought the XS adult size. My normal dress size is a 0.  This costume is rather baggy, but i pulled the belt way beyond where the velcro closure is and fastened it, then twisted it around behind me where the cape covers up how messy the belt is.  It works.  (it's for Halloween, not the prom!) :)  Also, i was just a little disappointed in the boot covers.  One is quite large and the other is TINY.  It is sized for about an 8 year old's foot/ankle.  I tore out the stitches in the back just to get it on.  (Again, it is just a cheesy boot cover and more to complete the look.)  I am considering cutting the whole foot part off it and wearing only the shaft part of the 'boot' with shoes, kind of like shiny leg warmers.
In any case, with all these little quirks of the costume, the price is great for a decent quality, very fun and cute costume. It is comfy too.