My son loves this costume, naturally.  He is 6, and generally wears a size 6 and is very slim, but at a normal height.  This costume is a Medium (8 to 10) and is supposed to fit a child aged 5 to 7.  It only fits well through the thighs and backside, (and he's skinny), but the costume has a lot of rubberized fitted pieces through the chest and shoulders and so it swallows him on top, mostly because it can't stretch or shrink to fit each individual child.  The included rubber belt and sleeve straps are a little large also.  Not sure whom this is supposed to fit?  A 6 year old who is 99th percentile in height, and 60% in weight I guess?  So, this will hopefully help you determine a size.  The helmet is fine, and like I said, he could care less about the exact fit and loves it.  I am hoping he grows taller, and stays slim for next year so he can wear it then.  Or you could get a set of long johns and velcro some shoulder pads at the shoulder to lengthen the torso and fill out the sholders better  We went ahead and got another costume locally that we could try on him (to fit better) and are saving this one.