Based on other reviews, which suggested that the fit on these was huge, I purchased the 6-12 month old size of this costume for my tall six month old baby who wears sizes 12 mos+ already.  I think it you were to fit it just snug, this would be accurate.  However, trying to put this on my baby, I realized why you'd want a bigger fit.  First of all, the costume is super stiff and has no give.  Second, the costume is pretty hard to put on.  It has a long zipper on the back (but not long enough to open the whole costume) and snaps at the crotch, but because of the stiffness of the fabric, it does not go on very easily.  It would really need to be zipped all the way down to the crotch (probably at the front) to work.  I tried to put this on my baby, who isn't a fan of having things put on over her head, and she HATED it.  She wouldn't let me put the arms on, so we took it off before we could check the size for fit.  After struggling to put this on for a mere few minutes, we were both sweaty and hot, and I believe it was from the non-breathable fabric.  Not sure how people make this work, and have their baby wear this for more than a few minutes without getting hot, sweaty and cranky!