We ordered size 12M-18M. I can't really fault the costume itself. It's very cute and would be great if it fit better, but you really can't know about fit unless you order it and try it on. The only thing that doesn't make me LOVE the costume is that the material is not breathable at all. I will be praying for a cooler night on Halloween. My son is 18M (as of yesterday) and he's not big for his age... probably 50th percentile, and this costume BARELY fits him. It was a bit of a struggle to get his arms into the costume, so I'm praying that he doesn't grow for a couple weeks. The booties don't fit him at all. They are VERY small. Probably would fit shoe size 4. The costume also has snaps all the way from one ankle to the top. If you have a very wiggly toddler, as I do, it's very hard to get him to stay still long enough to close all the snaps. Can't seem to find this costume in 2T. They just go from 18M to 3T... what the heck?! Anyway, here's the sizing information on the package (that they should include on the web site!):
HEIGHT: 28"-32"
CHEST: 19"-20"
WAIST: 19"-20"
WEIGHT: 20-26 lbs
100% Polyester. Hand wash cold separately. Do not bleach. Line dry. Cool iron if necessary. Do not dry clean. Hood is detachable.
Hope this helps!