My son is 3 years old and wears a size 3T.  He's about 34 lbs and 37 1/2 inches tall.  He is not tall, or heavy, actually is is on the smaller-side according to my pediatrician.  That said, I bought a size 4-6x, based on the advice in other reviews.  This is waaaay to small.  I will need to return it.  The jumpsuit/costume is exactly the right length for his height, but it is tight all over.  And because the fabric is so flimsy, my son would need to wear street clothes under it, which would make it even tighter.  Also, the ears hat is so small the ties wouldn't reach under his chin!  It looked ridiculous...Like it was made for a doll.  I am not happy with this costume and I will be returning it right away.  I advise not even trying it.  Don't buy it, if your little wants to dress up like Mickey Mouse, find another costume/brand.  This one is terrible.