I bought this costume for my 7-year-old son.  He loves it.  He wears a boys size 6 (He's 48 in. tall and 53 lbs) and the size Medium fits him generously. (Its a bit long and baggy, but I would not go down a size).  The quality of the costume is very good. I wish they included more than one mustache because if you use the tape on it once, it probably will not work very well for subsequent wearings.

Note to Luigi purists:  My older son informs me that the costume is not authentic because it makes Luigi appear chubby (there is an inflatable stomach included with the costume) and Luigi is supposed to be tall and thin (the opposite of Mario).  This does not seem to bother my 7-year-old -- he just omits the inflatable stomach.