I took measurements on my 4.5-year old, who is smaller than average, and based on the measurements, I bought the 3T set. It fit my slim son perfectly! The waist of the pants was a little snug, but all the lengths were perfect. The pants material is thin, but there is a silk shorts lining so you cant see through it. Its linen, so it wrinkles easily. Just iron and its ready to go!