"My son is barely 4 years old and also quite small for his age so I chose this toddler size instead of the next size up. Anyway, if you are in doubt, buy the next size up - we can get this one on my son but it is a very tight squeeze. Also, the mask is a nice idea - love that he can breathe - but it really doesn't cover his eyes. Still, he is thrilled to have his costume and it will certainly hold up for a single wearing."