This piece is beautiful, but unfortunately you cannot exchange if you get the wrong size.  I also ordered off my bra size and it was HUGE.  Nothing sexy about a baggy bustier - I'm hand-sewing it in discreet places to shrink it up a bit.  The back laces up, so if you end up ordering small, you can just loosen the lace.  Mine is big even with the lace as tight as it goes.  I'd recommend ordering a size down... it'll help make your boobs look bigger, too.

UPDATE: I began by just hand-sewing the boning over again to try to close up a few inches.  Turns out I needed way more than that.  I ended up taking in two whole sections (symmetrically).  Pretty much I took at least 6 inches out of this thing.  Possibly more like 8.  I should have ordered so many sizes smaller it's not even funny.  Also the thong that comes with this looks ridiculous (possibly because it's also super-huge).  It shows through the lace panel in the front.  Blah.