Without any sizing information, I decided to buy it anyway for my 18m old. It fits just fine (he wears a 2t). My 3 year old (in 3t) is a little too big for it, so I would guess this is for 18m-2t.
The jumpsuit goes on easily and fits well, it's the same material as all cheap costumes, but the inside isn't as itchy as most.
I was looking for a costume that wouldn't require my baby to wear individual pieces since he likely wouldn't keep them on (belts, gloves, foot covers, etc.). This fit the bill. The mask looks horrible, and has a big square "made in china" sticker on the side that goes against the face--and does not come off. So, even if it didn't look horrible it wouldn't wear very well. I'm ok with that because my baby won't wear a mask anyway, but if you wanted the mask, I'd look for a better option.