My three year old grandson was wearing size 6 boxers that were too big.  These Spiderman underoos are long brief style so they don't stick out of his pants and fit perfectly since he is a size 4 and you can't find the regular boxers in a size 4.  I gave this set 5 stars because he loves Spiderman, they fit well, and we had to order more so we could wash others for him to change!  I am adding that the shirt is quite small and does not fit like a 4T.  Overall I was more interested in the boxers than the shirt.  Had to pass the shirt down to a 2T size grandson.  Boxers still a 5 Star.