My 2 year old is small for his age, wears clothes for 12 to 18 months and 24 months depending on brand. As other reviewers have suggested, I went up a size and settled on the 3T. I've included a few photos, one showing the amount of shrinkage (machine washed and dried) and the other with a pair of 18 month old leggings for size comparison. The belt is almost the same size on all three leggings in the second photo.
ZukoCert Fleece Leggings Cashmere 2P B 130 Girls' Clothing for Leggings
61 Review
Adorable and comfortable Splendid girls' leggings for kids and babies
80 Review
Girls' Clothing and Leggings - Govc Winter Fleece Velvet Leggings
49 Review
IRELIA Girls Leggings Modal Spring Girls' Clothing for Leggings
52 Review