For size comparison: have a 12 month old, slightly on the smaller side (still wears some 9 month clothes), ordered the 12-18 mo. size. Fit as expected, slightly large, with room to grow, but wearable. We tested it out the other day at the pool (March in Texas). It worked fantastic to keep her covered, no sunburn, she seemed very comfortable, and extremely easy to get it off when wet (zipper spans all the way down to the ankle, so it comes off completely in one zip)! Expect it to be thin for breaSee full review