My son is 3 and wears a 4T or XS in boys. We ordered the small, and it fits perfect. Any bigger or smaller and it may not have worked.

Was very pleased with the quality of this costume. It does have some loose strings to trim, but for a halloween costume they don't get much better than this. Nice, thick fabric--not plastic-feeling at all. Looks warm for a night of trick or treating. And my son LOVES his new tail.

VERY HAPPY!