50 lb 6 yr old grand daughter loves this Cleo de Nile costume (size medium) - if she was any bigger it would be too small so if over 50 lbs or so I recommend going with large size.  Fun for Halloween - had to also purchase wig and make up to complete costume - cute for Halloween even though I'm not a fan of Monster High for little kids (must be getting conservative in my old age!).  She's worn it to play around the house a couple of times and it seems to be holding up well - should still look good for Halloween.