My son was wearing 12 month clothing when he wore this costume and it fit just fine and there was enough extra room that I would agree babies in the 18 month clothing sizes would fit as well.  I would agree with others that it could use a little extra bling and that it was a little long.  I didn't find the length to be so excessive that he couldn't wear the costume.  As a mother the only cautions I would give are that the entire outfit needs to come off for diaper changes and it's white so plan your parties accordingly (I had no problems machine washing it between events).  Sew on the cape and sash, they don't really stay in place.  I've also contemplating recycling this as an Evil Knieval costume since I didn't add the extra sequins to make it an obvious Elvis costume (a costume microphone acceossry worked just fine).