We purchased an xsmall size for our four-going-on-five year old son, who was wearing a 4t overall at the time.  The costume JUST fit.  The hand parts are very awkward to use, and made it hard for him to handle his candy bucket.  The head part fit well, but kept caving in on the top over and over.  I eventually gave up on fixing it.  BUT it was very cute, and it did fit for Halloween night, which was what really mattered.  My son enjoyed it, and finally got to be a T-Rex.