I had to guess on sizing since the (useless) sizing chart only lists 7, 8, etc but the item is sized S, M, L.  I ordered an XS for my son and it was too small.  After some searching the internet I found a site that lists some of the measurements:
XS 19-21" waist, S 22-24" waist, M 26-28" waist.

I had to reorder but I hope this is helpful for someone else!