My 8 month old wears mostly 12-18 month, so I got her the 12-24 month size, these tights look like they would fit a slender 3 year old. I bought them even though some of the reviews said they were tight because people also said that they were very stretchy, while this is true, they still left marks around my baby's tummy and thighs.  I'll post a picture of the tights next to some tights my baby fits into now so you can see the difference.