I made size medium using scraps of cotton knit material. It turned out bigger than I expected! I think these will probably fit a 6 year old as underwear, though they would probably fit as a diaper cover for a toddler as well.
I also found a home for wrong-size underwear such as this, at Project Underwear (http://www.projectunderwear.com/home.html). They provide 2 pairs of underwear to kids in poor countries who otherwise wouldn't have any. We take a lot of things for granted, and underwear is definitely one of them! These only took about 20 minutes to make, so I'll probably make more to donate.