I purchased this costume in a size SMALL for my 4-year-old's Halloween costume.  My son is approximately 40" and it was a good fit, not too large but a little roomy.  I can definitely see him wearing this to play Superman for a couple of years.  The arms and legs had a little extra material which he can grow into.  All in all size SMALL is a good size for children between the ages 4 to 6.  My one complaint was that the cape was coming apart at the seams, but with a little red thread I was able to fix the issue.