Everyone said this costume was too big but I didn't believe it.  I bought the smallest one available for my son who is 4 and half.  He is average size and has a spiderman costume from Costco and Target and it fits perfectly.  This one is waaaaay too big.  It is sloppy actually.  I would suggest if your son is 6 or 7 consider the small unless he is BIG.  The picture might show a small.  My son nevertheless loves the costume so that is the upside!